[repo] Simplify preprocessor directives after dropping support for .NET 6 and .NET7

[Kielek]

Fixes #5848
Follow up to #5712

Changes

Simplify preprocessor directives after dropping support for .NET 6 and .NET7

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 86.27%. Comparing base (6250307) to head (6a6a889).
Report is 320 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5851      +/-   ##
==========================================
+ Coverage   83.38%   86.27%   +2.88%     
==========================================
  Files         297      257      -40     
  Lines       12531    11168    -1363     
==========================================
- Hits        10449     9635     -814     
+ Misses       2082     1533     -549     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.13% <ø> (?)
unittests-Project-Stable	86.11% <ø> (?)
unittests-UnstableCoreLibraries-Experimental	85.60% <ø> (?)

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

Files with missing lines	Coverage Δ
...c/OpenTelemetry.Api/Logs/LogRecordAttributeList.cs	69.02% <ø> (+0.88%)	⬆️
src/OpenTelemetry.Api/Logs/LogRecordData.cs	100.00% <ø> (ø)
...nTelemetry.Api/Logs/LogRecordSeverityExtensions.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Logs/Logger.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Logs/LoggerProvider.cs	100.00% <ø> (ø)
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	72.50% <ø> (+2.50%)	⬆️
...try/Logs/ILogger/OpenTelemetryLoggingExtensions.cs	97.43% <ø> (+0.03%)	⬆️
src/OpenTelemetry/Logs/LogRecord.cs	70.77% <ø> (+1.04%)	⬆️
src/OpenTelemetry/Metrics/AggregatorStore.cs	87.46% <ø> (+7.08%)	⬆️
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	87.09% <ø> (-12.91%)	⬇️
... and 8 more

... and 210 files with indirect coverage changes

[CodeBlanch]

LGTM

[reyang]

This is great, thanks @Kielek!