[Instrumentation.SqlClient] 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 90.55%. Comparing base (71655ce) to head (66441c0).
Report is 492 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2159       +/-   ##
===========================================
+ Coverage   73.91%   90.55%   +16.64%     
===========================================
  Files         267        7      -260     
  Lines        9615      286     -9329     
===========================================
- Hits         7107      259     -6848     
+ Misses       2508       27     -2481     

Flag	Coverage Δ
unittests-Instrumentation.SqlClient	90.55% <ø> (?)

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see 271 files with indirect coverage changes