[repo] Bump OTelLatestStableVer to 1.9.0 and fix coreunstable test issues

[CodeBlanch]

Relates to #5695

Changes

• Sets OTelLatestStableVer in Directory.Packages.props to 1.9.0.
• Switches test projects for coreunstable- packages to use NuGet for core- packages when $(RunningDotNetPack) == true.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.13%. Comparing base (6250307) to head (7222b20).
Report is 261 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5696      +/-   ##
==========================================
+ Coverage   83.38%   86.13%   +2.74%     
==========================================
  Files         297      254      -43     
  Lines       12531    11054    -1477     
==========================================
- Hits        10449     9521     -928     
+ Misses       2082     1533     -549     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	85.75% <ø> (?)
unittests-Project-Stable	86.01% <ø> (?)
unittests-Solution	86.05% <ø> (?)
unittests-UnstableCoreLibraries-Experimental	84.54% <ø> (?)

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see 124 files with indirect coverage changes