Cleanup .NET 5.0

[cijothomas]

Fixes #3147
Final PR.

[codecov]

Codecov Report

Merging #3227 (c39f265) into main (0868872) will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3227      +/-   ##
==========================================
- Coverage   85.58%   85.33%   -0.25%     
==========================================
  Files         261      261              
  Lines        9390     9390              
==========================================
- Hits         8036     8013      -23     
- Misses       1354     1377      +23     

Impacted Files	Coverage Δ
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	50.00% <0.00%> (-28.58%)	⬇️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	59.09% <0.00%> (-18.19%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	73.52% <0.00%> (-5.89%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	88.73% <0.00%> (-3.61%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	84.21% <0.00%> (-3.16%)	⬇️
...heus/Implementation/PrometheusCollectionManager.cs	79.74% <0.00%> (-2.54%)	⬇️