[release] Core stable release 1.12.0 updates

[opentelemetrybot]

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 1.12.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.45%. Comparing base (a67acd8) to head (985bea3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6269   +/-   ##
=======================================
  Coverage   86.44%   86.45%           
=======================================
  Files         258      258           
  Lines       11794    11794           
=======================================
+ Hits        10195    10196    +1     
+ Misses       1599     1598    -1     

Flag	Coverage Δ
unittests-Project-Experimental	86.33% <ø> (+0.22%)	⬆️
unittests-Project-Stable	86.38% <ø> (+<0.01%)	⬆️
unittests-Solution	86.10% <ø> (-0.04%)	⬇️
unittests-UnstableCoreLibraries-Experimental	85.87% <ø> (+0.09%)	⬆️

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see 5 files with indirect coverage changes

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