[repo/ElasticsearchClient] 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 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.12%. Comparing base (71655ce) to head (2640277).
Report is 580 commits behind head on main.

Files with missing lines	Patch %	Lines
.../ElasticsearchRequestPipelineDiagnosticListener.cs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2292      +/-   ##
==========================================
+ Coverage   73.91%   80.12%   +6.20%     
==========================================
  Files         267        5     -262     
  Lines        9615      161    -9454     
==========================================
- Hits         7107      129    -6978     
+ Misses       2508       32    -2476     

Flag	Coverage Δ
unittests-Instrumentation.ElasticsearchClient	80.12% <50.00%> (?)

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

Files with missing lines	Coverage Δ
.../ElasticsearchRequestPipelineDiagnosticListener.cs	83.73% <50.00%> (+1.53%)	⬆️

... and 263 files with indirect coverage changes

[CodeBlanch]

LGTM