[Exporter.Geneva] 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 59.12%. Comparing base (71655ce) to head (96d88eb).
Report is 457 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2117       +/-   ##
===========================================
- Coverage   73.91%   59.12%   -14.80%     
===========================================
  Files         267       44      -223     
  Lines        9615     3924     -5691     
===========================================
- Hits         7107     2320     -4787     
+ Misses       2508     1604      -904     

Flag	Coverage Δ
unittests-Exporter.Geneva	59.12% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...xporter.Geneva/Internal/MsgPack/MsgPackExporter.cs	93.93% <ø> (ø)
...rter.Geneva/Internal/MsgPack/MsgPackLogExporter.cs	94.44% <ø> (ø)
...er.Geneva/Internal/MsgPack/MsgPackTraceExporter.cs	94.95% <ø> (ø)
...rter.Geneva/Internal/Tld/UncheckedASCIIEncoding.cs	17.30% <ø> (ø)
...ry.Exporter.Geneva/Metrics/GenevaMetricExporter.cs	96.96% <ø> (+30.30%)	⬆️

... and 247 files with indirect coverage changes

[CodeBlanch]

LGTM