Support .NET 11

[martincostello]

The intention is to keep this PR updated over the course of .NET 11's development as new previews are released.

Changes

Add support for building and testing with .NET 11.

• Build with .NET 11 SDK
• Add net11.0 target
• Drop net8.0 and net9.0 targets

TODO

• Use stable (or RC) SDK and NuGet package versions
• Use fully-qualified container image tags (i.e. not 11.0-preview)
• Remove workarounds for .NET 11 preview issues

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

❌ Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.18%. Comparing base (8592991) to head (b802571).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...penTelemetryProtocol/PersistentStorage/FileBlob.cs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6899      +/-   ##
==========================================
- Coverage   90.21%   90.18%   -0.04%     
==========================================
  Files         287      287              
  Lines       15560    15575      +15     
==========================================
+ Hits        14038    14046       +8     
- Misses       1522     1529       +7     

Flag	Coverage Δ
unittests-PowerShellScripts	88.49% <ø> (ø)
unittests-Project-Experimental	90.18% <86.11%> (-0.08%)	⬇️
unittests-Project-Stable	90.12% <86.11%> (-0.09%)	⬇️
unittests-Solution	90.06% <86.11%> (-0.18%)	⬇️
unittests-UnstableCoreLibraries-Experimental	50.57% <50.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
...nTelemetry.Api/Context/Propagation/B3Propagator.cs	89.83% <100.00%> (ø)
...metry.Api/Context/Propagation/BaggagePropagator.cs	93.78% <100.00%> (ø)
....Api/Context/Propagation/TraceContextPropagator.cs	90.49% <100.00%> (ø)
...c/OpenTelemetry.Api/Logs/LogRecordAttributeList.cs	74.78% <100.00%> (ø)
...yProtocol/Implementation/OtlpCertificateManager.cs	76.47% <ø> (-0.18%)	⬇️
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	99.10% <100.00%> (+<0.01%)	⬆️
...tocol/PersistentStorage/PersistentStorageHelper.cs	48.86% <100.00%> (+5.82%)	⬆️
.../PrometheusExporterApplicationBuilderExtensions.cs	100.00% <100.00%> (ø)
...rometheusExporterEndpointRouteBuilderExtensions.cs	100.00% <100.00%> (ø)
...ner/Internal/Shared/PrometheusCollectionManager.cs	90.37% <100.00%> (ø)
... and 7 more

... and 4 files with indirect coverage changes

[martincostello]

Commit that should fix all the warnings just pushed. Will make a version targeting main tomorrow that doesn't contain C#-15-specific features (like [with(...)])- those specific changes can be reviewed and refined there (and potentially later broken up into smaller PRs by project).