[Exporter.InfluxDB] Replace .NET 6 target with .NET 8

[joegoldman2]

Changes

Replace .NET 6 target, which will be very soon out of support, with .NET 8.

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 95.88%. Comparing base (71655ce) to head (770ecb3).
Report is 459 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2116       +/-   ##
===========================================
+ Coverage   73.91%   95.88%   +21.96%     
===========================================
  Files         267        7      -260     
  Lines        9615      243     -9372     
===========================================
- Hits         7107      233     -6874     
+ Misses       2508       10     -2498     

Flag	Coverage Δ
unittests-Exporter.InfluxDB	95.88% <ø> (?)

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see 264 files with indirect coverage changes

[CodeBlanch]

LGTM