[Instrumentation.Http] 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 73.57%. Comparing base (71655ce) to head (4e5dfdb).
Report is 486 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2152      +/-   ##
==========================================
- Coverage   73.91%   73.57%   -0.35%     
==========================================
  Files         267       11     -256     
  Lines        9615      840    -8775     
==========================================
- Hits         7107      618    -6489     
+ Misses       2508      222    -2286     

Flag	Coverage Δ
unittests-Instrumentation.Http	73.57% <ø> (?)

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see 275 files with indirect coverage changes