[Instrumentation.GrpcNetClient] Remove .NET 6 target

[joegoldman2]

Changes

Remove .NET 6 target which will be very soon out of support.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.61%. Comparing base (71655ce) to head (f5c6215).
Report is 479 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2150      +/-   ##
==========================================
+ Coverage   73.91%   79.61%   +5.69%     
==========================================
  Files         267        5     -262     
  Lines        9615      103    -9512     
==========================================
- Hits         7107       82    -7025     
+ Misses       2508       21    -2487     

Flag	Coverage Δ
unittests-Instrumentation.GrpcNetClient	79.61% <ø> (?)

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see 269 files with indirect coverage changes