Fix TFM bugs

[reyang]

Fix two issues:

1. the TFM comparsion logic should use != true instead of == false since in certain environments the property doesn't exist.
2. the System.Net.Http reference should be coming from framework instead of package, @cijothomas explained to me that https://www.nuget.org/packages/System.Net.Http/ nuget should not be used at all because it's abandoned since year 2018.

[reyang]

This shouldn't be used at all since it's a dead package https://www.nuget.org/packages/System.Net.Http/ (no updates since year 2018).

[codecov]

Codecov Report

Merging #4912 (3085fb3) into main (c18ff65) will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    open-telemetry/opentelemetry-dotnet#4912      +/-   ##
==========================================
+ Coverage   83.30%   83.51%   +0.20%     
==========================================
  Files         295      295              
  Lines       12294    12294              
==========================================
+ Hits        10242    10267      +25     
+ Misses       2052     2027      -25     

Flag	Coverage Δ
unittests	83.51% <ø> (+0.20%)	⬆️

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

see 6 files with indirect coverage changes

[CodeBlanch]

I'm going to approve this but FYI what I really want is for the tests to verify everything works if the user decides to take the NuGet bits. But we should also make sure it works with the direct reference. Probably need to run the tests through both modes to do it correctly? Going to take a bit of effort to pull that off, not sure if the juice is worth the squeeze.

[reyang]

With this PR, the test is going to cover the net462 framework reference of System.Net.Http, do you think such test coverage can represent NuGet System.Net.Http (especially considering that the NuGet hasn't been updated since 2018) or we need both?

what I really want is for the tests to verify everything works if the user decides to take the NuGet bits

Could you explain a bit more here? What's the main scenario and how big is the user base?

[CodeBlanch]

It is definitely old, but I don't see anything on the NuGet saying it is deprecated and it has been downloaded 1.7B times so a lot of people are consuming it! The scenario is user takes a dependency on the NuGet and its bits are used instead of the shipped runtime assembly,

[reyang]

#4915