Use DiagnosticSource 7.0 and drop netcoreapp3.1 target

[alanwest]

Merging this to a new net7.0 branch for so we have a place to start work that is dependent on DiagnosticSource 7.0.

• UpDownCounter support
• Using the new enumerator APIs e.g., https://docs.microsoft.com/dotnet/api/system.diagnostics.activity.enumeratetagobjects
• Test new ASP.NET Core and HttpClient ActivitySource instrumentation

[cijothomas]

lets re-enable CI for this branch as well.

[codecov]

Codecov Report

Merging #3351 (ef1d7ed) into net7.0 (efc55d5) will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           net7.0    #3351      +/-   ##
==========================================
- Coverage   85.90%   85.70%   -0.20%     
==========================================
  Files         267      267              
  Lines        9383     9368      -15     
==========================================
- Hits         8060     8029      -31     
- Misses       1323     1339      +16     

Impacted Files	Coverage Δ
...mentation/ExportClient/ExporterClientValidation.cs	30.00% <0.00%> (-70.00%)	⬇️
src/OpenTelemetry.Api/BaseProvider.cs	66.66% <0.00%> (-33.34%)	⬇️
...c/OpenTelemetry/Internal/TagAndValueTransformer.cs	80.00% <0.00%> (-20.00%)	⬇️
...Propagators/OpenTelemetryPropagatorsEventSource.cs	87.50% <0.00%> (-12.50%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
...rc/OpenTelemetry.Exporter.Zipkin/ZipkinExporter.cs	86.17% <0.00%> (-2.26%)	⬇️
...Exporter.Jaeger/ApacheThrift/Protocol/TProtocol.cs	91.30% <0.00%> (-0.86%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️
...er.Jaeger/ApacheThrift/Protocol/TBinaryProtocol.cs	78.88% <0.00%> (-0.69%)	⬇️
... and 3 more

[alanwest]

lets re-enable CI for this branch as well.

Enabled and passing. Going to merge.