remove netcoreapp2.1 from source

[TimothyMothra]

Fix Issue #2251

Changes

• Remove netcoreapp2.1 and older from source.
• Found some code that doesn't compile. I marked it with #TODO

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

Notes for reviewers:

• We support comment build triggers
  • /AzurePipelines run will queue all builds
  • /AzurePipelines run <pipeline-name> will queue a specific build

[cijothomas]

suggest using the 2.1*

[TimothyMothra]

why 2.1 here as opposed to latest?
I have no problem with the downgrade, just trying to understand :)

[cijothomas]

if its testing, yes you can use latest.

[rajkumar-rangaraj]

If we are planning to cover .NET 5.0, .NET 6.0+, then better to include NET along with NETCOREAPP. Ref: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#conditional-compilation

[TimothyMothra]

NET and NETCOREAPP both cover the same frameworks.

In my opinion, NET sounds ambiguous between the older NetFrameworks (4.5 - 4.8).
I would prefer to use NETCOREAPP to differentiate.

[cijothomas]

since NETCOREAPP covers NET5.0 and higher, this is fine i think.