[repo/StackDriver] 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

Attention: Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.26%. Comparing base (71655ce) to head (d8e9f1d).
Report is 559 commits behind head on main.

Files with missing lines	Patch %	Lines
...r.Stackdriver/Implementation/ActivityExtensions.cs	41.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2270      +/-   ##
==========================================
+ Coverage   73.91%   79.26%   +5.35%     
==========================================
  Files         267        8     -259     
  Lines        9615      164    -9451     
==========================================
- Hits         7107      130    -6977     
+ Misses       2508       34    -2474     

Flag	Coverage Δ
unittests-Exporter.Stackdriver	79.26% <58.82%> (?)

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

Files with missing lines	Coverage Δ
...y.Exporter.Stackdriver/StackdriverTraceExporter.cs	78.57% <100.00%> (+11.07%)	⬆️
...r.Stackdriver/Implementation/ActivityExtensions.cs	88.88% <41.66%> (+13.88%)	⬆️

... and 263 files with indirect coverage changes

[CodeBlanch]

LGTM