Use EnableCodeStyleSeverity instead of AnalysisLevel to control new diagnostic severity behavior

[sharwell]

• Use built-in features for marking unnecessary code
• Verify missing code style severity falls back to old behavior
• Use EnableCodeStyleSeverity instead of AnalysisLevel to control new diagnostic severity behavior

Follow-up to #72238

[cremor]

If I understand this PR correctly then these documentation pages need to be updated for the change, correct?
https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/overview#enable-on-build
https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/language-rules#option-format

[sharwell]

Yes, those would need to be updated. We'll also need to update the SDK code style build targets to feed the new build property to the compiler.