Migrate to global.json for .NET usage in our templates

[konrad-jamrozik]

Per @weshaggard's comment here:

• Bump globals.yml .NET SDK version from 3.1.405 to 6.x + assorted changes #4916 (comment)

We should migrate our usage of .NET to a solution based on global.json, and a pipeline definition like install-dotnet.yml, instead of relying on globals.yml template variables used to install .NET by our archetypes, as e.g. seen in archetype-sdk-tool-dotnet.yml#L70.

Changes to be made

• As an end result, we don't want to use UseDotNet task anywhere except install-net.yml.
  • At the very least we should migrate all archetype- files to it.
• We should evaluate every feature used by azure-sdk-for-net install-net.yml for applicability to our case, and if relevant, transplant it.
• We should consider updating the msbuild-sdks entry already present in our global.json.

Additional details

• Here is the azure-sdk-for-net repo global.json.
• The pipeline definition install-dotnet.yml refers to the global.json on line 18 via useGlobalJson.
• The install-dotnet.yml is used in many pipeline definitions in pipelines/templates/steps, as a template.
• This work item was originally proposed while working on fixing:
  • The template for prepare-pipelines is broken by the move to ubuntu-22.04 #4888