Remove netstandard targets from asp.netcore instrumentation

[vishweshbankwar]

Fixes #.

Changes

As discussed during SIG meeting, removing netstandard2.0 and netstandard2.1 targets from the library.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #3567 (50fac81) into main (28473bc) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3567      +/-   ##
==========================================
- Coverage   87.38%   87.23%   -0.15%     
==========================================
  Files         278      278              
  Lines       10088    10088              
==========================================
- Hits         8815     8800      -15     
- Misses       1273     1288      +15     

Impacted Files	Coverage Δ
...tation.AspNetCore/Implementation/HttpInListener.cs	90.18% <ø> (ø)
...AspNetCore/Implementation/HttpInMetricsListener.cs	65.71% <ø> (ø)
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	35.71% <0.00%> (-42.86%)	⬇️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	36.36% <0.00%> (-40.91%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	73.52% <0.00%> (-5.89%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	91.20% <0.00%> (-4.40%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	76.66% <0.00%> (-0.84%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	84.11% <0.00%> (+1.86%)	⬆️
src/OpenTelemetry/Logs/OpenTelemetryLogger.cs	88.88% <0.00%> (+2.22%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	80.48% <0.00%> (+2.43%)	⬆️
... and 3 more

[alanwest]

LGTM

[vishweshbankwar]

@utpilla - Can we merge this one before your release?