[release] Core stable release 1.13.1 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 1.13.1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.63%. Comparing base (19dda39) to head (fecf8fa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6598      +/-   ##
==========================================
- Coverage   86.69%   86.63%   -0.06%     
==========================================
  Files         258      258              
  Lines       11910    11910              
==========================================
- Hits        10325    10318       -7     
- Misses       1585     1592       +7     

Flag	Coverage Δ
unittests-Project-Experimental	86.37% <ø> (+0.09%)	⬆️
unittests-Project-Stable	86.55% <ø> (ø)
unittests-Solution	86.60% <ø> (-0.06%)	⬇️
unittests-UnstableCoreLibraries-Experimental	85.87% <ø> (ø)

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see 1 file with indirect coverage changes