[release] Core stable release 1.15.1 updates

[otelbot-dotnet]

Note: This PR was opened automatically by the post-release workflow.

Merge once packages are available on NuGet and the build passes.

Changes

• Sets OTelLatestStableVer in Directory.Packages.props to 1.15.1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.85%. Comparing base (a065d9f) to head (a1da35b).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7010      +/-   ##
==========================================
- Coverage   88.87%   88.85%   -0.03%     
==========================================
  Files         263      263              
  Lines       12424    12424              
==========================================
- Hits        11042    11039       -3     
- Misses       1382     1385       +3     

Flag	Coverage Δ
unittests-Project-Experimental	88.79% <ø> (-0.06%)	⬇️
unittests-Project-Stable	88.72% <ø> (-0.09%)	⬇️
unittests-Solution	88.78% <ø> (+0.08%)	⬆️
unittests-UnstableCoreLibraries-Experimental	42.40% <ø> (ø)

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see 4 files with indirect coverage changes