Bump AnalysisLevel to 8

[Morilli]

This bumps the AnalysisLevel for code analyzers from 6 to 8, enabling more analyzers by default. This is now safe as we require the .NET 8 SDK to build.

I've applied fixes and severity changes to all new inspections. Open to feedback on whether any of those are disagreed on.

Check if completed:

• [x] I have run any relevant test suites
• [x] I, the committer, have read the licensing terms for contributors (last updated 2024-06-22) and am compliant