[repo/OneCollector] 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 98.14815% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.50%. Comparing base (71655ce) to head (539696a).
Report is 612 commits behind head on main.

Files with missing lines	Patch %	Lines
...ternal/Serialization/CommonSchemaJsonSerializer.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2341       +/-   ##
===========================================
+ Coverage   73.91%   94.50%   +20.58%     
===========================================
  Files         267       28      -239     
  Lines        9615     1183     -8432     
===========================================
- Hits         7107     1118     -5989     
+ Misses       2508       65     -2443     

Flag	Coverage Δ
unittests-Exporter.OneCollector	94.50% <98.14%> (?)

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

Files with missing lines	Coverage Δ
....Exporter.OneCollector/Internal/CallbackManager.cs	96.00% <100.00%> (+0.16%)	⬆️
...er.OneCollector/Internal/ConnectionStringParser.cs	100.00% <100.00%> (ø)
...ry.Exporter.OneCollector/Internal/EventFullName.cs	100.00% <100.00%> (ø)
...Exporter.OneCollector/Internal/EventNameManager.cs	100.00% <100.00%> (+2.85%)	⬆️
...rialization/CommonSchemaJsonSerializationHelper.cs	100.00% <100.00%> (ø)
...erialization/CommonSchemaJsonSerializationState.cs	97.36% <100.00%> (+0.10%)	⬆️
...rialization/LogRecordCommonSchemaJsonSerializer.cs	99.21% <100.00%> (+0.04%)	⬆️
...tor/Internal/Sinks/WriteDirectlyToTransportSink.cs	97.33% <100.00%> (+6.19%)	⬆️
...ector/Internal/Transports/HttpJsonPostTransport.cs	95.62% <100.00%> (+0.20%)	⬆️
...neCollector/Logs/OneCollectorLogExporterOptions.cs	100.00% <100.00%> (+20.00%)	⬆️
... and 2 more

... and 255 files with indirect coverage changes

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[CodeBlanch]

LGTM