[otlp] Remove AppContext verification when using grpc + insecure endpoints

[Kielek]

Fixes N/A
Design discussion issue N/A

Changes

Drop all references to System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport. It was needed for .NET Core 3.x only. Now, only .NET6+ is supported so the code can be removed.

For sure 2 first commits can be merged. I have some doubts about 7379247. Formally, there is no support to .NET Core 3.x in this repository but exporter package is targeted to netstandard2.0 and netstndard2.1. Both can be technically referenced by .NET Core 3.x. Not supported per discsion in this PR.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes N/A
• [ ] Changes in public API reviewed (if applicable) N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.53%. Comparing base (6250307) to head (c607314).
Report is 153 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5486      +/-   ##
==========================================
+ Coverage   83.38%   85.53%   +2.14%     
==========================================
  Files         297      289       -8     
  Lines       12531    12593      +62     
==========================================
+ Hits        10449    10771     +322     
+ Misses       2082     1822     -260     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.51% <ø> (?)
unittests-Solution-Stable	85.62% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...mentation/ExportClient/BaseOtlpGrpcExportClient.cs	78.94% <ø> (-5.27%)	⬇️

... and 62 files with indirect coverage changes

[pellared]

🏅

[vishweshbankwar]

LGTM - @Kielek Could you please update the PR description with the link https://learn.microsoft.com/en-us/aspnet/core/grpc/troubleshoot?view=aspnetcore-8.0#call-insecure-grpc-services-with-net-core-client

Regarding the question on using netstandard2.0 with netcoreapp3.1, we do not support this scenario. #3767

[Kielek]

LGTM - @Kielek Could you please update the PR description with the link https://learn.microsoft.com/en-us/aspnet/core/grpc/troubleshoot?view=aspnetcore-8.0#call-insecure-grpc-services-with-net-core-client

Done.

Regarding the question on using netstandard2.0 with netcoreapp3.1, we do not support this scenario. #3767

Great! All commits can be merged.