[repo] CI updates

[CodeBlanch]

Applying some CI updates from contrib (open-telemetry/opentelemetry-dotnet-contrib#827) to main repo.

• Set SDK to 7.0.101 in global.json so that everyone has the code analysis fix.
• Integration tests will now build on net7.0 SDK but run on the target SDK versions. Closer to real world usage.
• A bit of cleanup.

[codecov]

Codecov Report

Merging #4016 (f059530) into main (9ad312a) will increase coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4016      +/-   ##
==========================================
+ Coverage   85.49%   85.52%   +0.03%     
==========================================
  Files         289      289              
  Lines       11229    11229              
==========================================
+ Hits         9600     9604       +4     
+ Misses       1629     1625       -4     

Impacted Files	Coverage Δ
...Propagators/OpenTelemetryPropagatorsEventSource.cs	87.50% <0.00%> (-12.50%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...Listener/Internal/PrometheusExporterEventSource.cs	66.66% <0.00%> (+5.55%)	⬆️
...tation/OpenTelemetryProtocolExporterEventSource.cs	95.00% <0.00%> (+10.00%)	⬆️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	86.36% <0.00%> (+13.63%)	⬆️