Update to recommended target framework moniker (TFM) to net6.0

[scottschreckengaust]

Describe the feature

Use recent dotnet TargetFrameworks, https://github.com/dotnet/core:

https://learn.microsoft.com/en-us/dotnet/standard/frameworks

• net6.0
• net8.0

Use Case

Although the netcoreapp3.1 target is still a "Latest version", https://learn.microsoft.com/en-us/dotnet/standard/frameworks#latest-versions, the net6.0 and net8.0 are within the current dotnet support frameworks.

Proposed Solution

Options:

1. Update

   jsii/packages/jsii-pacmak/lib/targets/dotnet.ts

   Line 21 in 4ee891d

   export const TARGET_FRAMEWORK = 'netcoreapp3.1';

   to net6.0 and other requirements as needed.

2. Apply <TargetFrameworks>netcoreapp3.1;net6.0</TargetFrameworks> and optionally add <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> to remove warnings:

   Although there is a risk in the warning hints:

   ...You may also set true in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk.

Other Information

Builds have warnings:

doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later.

A previous attempt #3957 and rollback #3987 exist.

Here are some examples from the warnings:

warning : Microsoft.Extensions.Configuration 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Configuration.Abstractions 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Configuration.Binder doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.DependencyInjection 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.DependencyInjection.Abstractions 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Logging 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Logging.Abstractions 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Logging.Configuration 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Logging.Console 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Options 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Options.ConfigurationExtensions 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : Microsoft.Extensions.Primitives 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : System.Diagnostics.DiagnosticSource 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : System.Text.Encodings.Web 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 
warning : System.Text.Json 8.0.0 doesn't support netcoreapp3.1 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own 

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

CDK version used

2.149.0

Environment details (OS name and version, etc.)

macOS Sonoma 14.5