Add net7.0 target to test and docs project

[vishweshbankwar]

Fixes #3840

Changes

Please provide a brief description of the changes here.

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 #3842 (814c958) into main (c7dff42) will decrease coverage by 0.15%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3842      +/-   ##
==========================================
- Coverage   87.57%   87.41%   -0.16%     
==========================================
  Files         280      280              
  Lines       10765    10765              
==========================================
- Hits         9427     9410      -17     
- Misses       1338     1355      +17     

Impacted Files	Coverage Δ
...tation/OpenTelemetryProtocolExporterEventSource.cs	85.00% <0.00%> (-15.00%)	⬇️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	72.72% <0.00%> (-13.64%)	⬇️
...lemetry/Internal/SelfDiagnosticsConfigRefresher.cs	86.53% <0.00%> (-5.77%)	⬇️
...Listener/Internal/PrometheusExporterEventSource.cs	61.11% <0.00%> (-5.56%)	⬇️
...lementation/SqlClientInstrumentationEventSource.cs	70.83% <0.00%> (-4.17%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	82.24% <0.00%> (-1.87%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.15% <0.00%> (-1.59%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+2.94%)	⬆️
... and 2 more

[cijothomas]

Thanks.
we'll need to some of the nuget versions to a common place for easy upgrade next time.