Improve <LangVersion> selection

[paulmedynski]

Improved the C# language version selection:

• For .NET, let the SDK choose the best language version.
• For everything else (Framework, Standard), explicitly choose C# 13.

I used the #error version preprocessor directive to validate that the expected C# version was being used for the MDS projects and the test projects.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.73%. Comparing base (1247ca4) to head (38b2cb3).
Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3240      +/-   ##
==========================================
+ Coverage   72.69%   74.73%   +2.04%     
==========================================
  Files         303      314      +11     
  Lines       59718    75258   +15540     
==========================================
+ Hits        43414    56247   +12833     
- Misses      16304    19011    +2707     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	76.52% <ø> (+1.41%)	⬆️
netfx	73.19% <ø> (+1.81%)	⬆️

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• src/Directory.Build.props: Language not supported

[paulmedynski]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[paulmedynski]

We can take this approach as implemented, or we can set to latest for all targets. I think I prefer the implemented approach for now. The latest approach would require us to ensure we lock down our SDKs on all build agents/images/pipelines. It may be the better long-term approach though.