[Instrumentation.AspNetCore] metrics - cleanup code after dropping support for .NET6

[Kielek]

Changes

[Instrumentation.AspNetCore] metrics - cleanup code after dropping support for .NET6
It removes functionality even from neststandard2.0 target.

Inspired by open-telemetry/opentelemetry-dotnet-instrumentation#3861

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 78.29%. Comparing base (71655ce) to head (9b13002).
Report is 639 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2360      +/-   ##
==========================================
+ Coverage   73.91%   78.29%   +4.38%     
==========================================
  Files         267       22     -245     
  Lines        9615      622    -8993     
==========================================
- Hits         7107      487    -6620     
+ Misses       2508      135    -2373     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	76.73% <ø> (?)
unittests-Instrumentation.AspNetCore	80.45% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...reInstrumentationMeterProviderBuilderExtensions.cs	100.00% <100.00%> (ø)

... and 273 files with indirect coverage changes