[repo/InfluxDB] Prepare to .NET9

[Kielek]

Changes

Follow up #2251

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

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.14%. Comparing base (71655ce) to head (11077d0).
Report is 556 commits behind head on main.

Files with missing lines	Patch %	Lines
...ry.Exporter.InfluxDB/InfluxDBExporterExtensions.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2268       +/-   ##
===========================================
+ Coverage   73.91%   95.14%   +21.22%     
===========================================
  Files         267        7      -260     
  Lines        9615      247     -9368     
===========================================
- Hits         7107      235     -6872     
+ Misses       2508       12     -2496     

Flag	Coverage Δ
unittests-Exporter.InfluxDB	95.14% <83.33%> (?)

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

Files with missing lines	Coverage Δ
...metry.Exporter.InfluxDB/InfluxDBMetricsExporter.cs	85.71% <100.00%> (-2.75%)	⬇️
...ry.Exporter.InfluxDB/TelegrafPrometheusWriterV1.cs	100.00% <100.00%> (ø)
...ry.Exporter.InfluxDB/TelegrafPrometheusWriterV2.cs	100.00% <100.00%> (ø)
...ry.Exporter.InfluxDB/InfluxDBExporterExtensions.cs	95.12% <50.00%> (-4.88%)	⬇️

... and 260 files with indirect coverage changes

[CodeBlanch]

LGTM