[Instrumentation.AspNetCore] 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 73.83%. Comparing base (71655ce) to head (463924e).
Report is 477 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2138      +/-   ##
==========================================
- Coverage   73.91%   73.83%   -0.09%     
==========================================
  Files         267       23     -244     
  Lines        9615      665    -8950     
==========================================
- Hits         7107      491    -6616     
+ Misses       2508      174    -2334     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	77.00% <ø> (?)
unittests-Instrumentation.AspNetCore	70.06% <ø> (?)

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see 276 files with indirect coverage changes