[repo/Extensions] 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 87.29%. Comparing base (71655ce) to head (3451d26).
Report is 560 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2265       +/-   ##
===========================================
+ Coverage   73.91%   87.29%   +13.37%     
===========================================
  Files         267       50      -217     
  Lines        9615     1149     -8466     
===========================================
- Hits         7107     1003     -6104     
+ Misses       2508      146     -2362     

Flag	Coverage Δ
unittests-Extensions	88.63% <ø> (?)
unittests-Extensions.Enrichment	100.00% <100.00%> (?)
unittests-Instrumentation.AWS	86.06% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...OpenTelemetry.Extensions.AWS/AWSXRayIdGenerator.cs	93.15% <100.00%> (+32.03%)	⬆️
...elemetry.Extensions.AWS/Trace/AWSXRayPropagator.cs	79.83% <100.00%> (+0.32%)	⬆️
...ions.Enrichment/Internal/TraceEnrichmentActions.cs	100.00% <100.00%> (ø)

... and 281 files with indirect coverage changes

[CodeBlanch]

LGTM