[release] Core stable release 1.15.0 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 1.15.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.91%. Comparing base (164d8a5) to head (6af6a63).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6841      +/-   ##
==========================================
- Coverage   86.92%   86.91%   -0.01%     
==========================================
  Files         263      263              
  Lines       12382    12382              
==========================================
- Hits        10763    10762       -1     
- Misses       1619     1620       +1     

Flag	Coverage Δ
unittests-Project-Experimental	86.72% <ø> (-0.11%)	⬇️
unittests-Project-Stable	86.83% <ø> (+0.04%)	⬆️
unittests-Solution	86.74% <ø> (-0.17%)	⬇️
unittests-UnstableCoreLibraries-Experimental	86.07% <ø> (ø)

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see 4 files with indirect coverage changes