Remove NET5.0 target from OTLP Exporters

[cijothomas]

Towards #3147

[codecov]

Codecov Report

Merging #3226 (6bd6d85) into main (d1ec45c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3226      +/-   ##
==========================================
- Coverage   85.56%   85.55%   -0.02%     
==========================================
  Files         261      261              
  Lines        9390     9390              
==========================================
- Hits         8035     8034       -1     
- Misses       1355     1356       +1     

Impacted Files	Coverage Δ
...mentation/ExportClient/BaseOtlpGrpcExportClient.cs	83.33% <ø> (ø)
...mentation/ExportClient/BaseOtlpHttpExportClient.cs	100.00% <ø> (ø)
...ementation/ExportClient/OtlpHttpLogExportClient.cs	0.00% <ø> (ø)
...tation/ExportClient/OtlpHttpMetricsExportClient.cs	62.50% <ø> (ø)
...entation/ExportClient/OtlpHttpTraceExportClient.cs	87.50% <ø> (ø)
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	97.87% <ø> (ø)
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️