[repo/AspNet] Prepare to .NET9

[Kielek]

Changes

Follow up #2251

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [ ] Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.73%. Comparing base (71655ce) to head (158d7d5).
Report is 571 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2282      +/-   ##
==========================================
+ Coverage   73.91%   76.73%   +2.81%     
==========================================
  Files         267       15     -252     
  Lines        9615      361    -9254     
==========================================
- Hits         7107      277    -6830     
+ Misses       2508       84    -2424     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	76.73% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 268 files with indirect coverage changes

[CodeBlanch]

LGTM