[release] Core stable release 1.15.3 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 1.15.3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.07%. Comparing base (c0689a3) to head (b1a1254).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7125      +/-   ##
==========================================
- Coverage   89.11%   89.07%   -0.05%     
==========================================
  Files         271      271              
  Lines       13068    13065       -3     
==========================================
- Hits        11645    11637       -8     
- Misses       1423     1428       +5     

Flag	Coverage Δ
unittests-Project-Experimental	88.86% <ø> (-0.16%)	⬇️
unittests-Project-Stable	89.03% <ø> (-0.07%)	⬇️
unittests-Solution	89.03% <ø> (+0.02%)	⬆️
unittests-UnstableCoreLibraries-Experimental	40.76% <ø> (-0.66%)	⬇️

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see 5 files with indirect coverage changes