fix: IsOsPlatform() fails on older .NET Framework Versions

[tippmar-nr]

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Description

Moved all calls to RuntimeInformation.IsOsPlatform() within a #if NETSTANDARD2_0 block.

Author Checklist

• Unit tests, Integration tests, and Unbounded tests completed
• Performance testing completed with satisfactory results (if required)

Reviewer Checklist

• Perform code review
• Pull request was adequately tested (new/existing tests, performance tests)

[codecov-commenter]

Codecov Report

Merging #1552 (6f28d01) into main (e49bf63) will increase coverage by 0.00%.
The diff coverage is 12.50%.

@@           Coverage Diff           @@
##             main    #1552   +/-   ##
=======================================
  Coverage   73.03%   73.03%           
=======================================
  Files         408      408           
  Lines       25575    25576    +1     
=======================================
+ Hits        18678    18679    +1     
  Misses       6897     6897           

Impacted Files	Coverage Δ
...elic/Agent/Core/NewRelic.Agent.Core/Environment.cs	56.72% <0.00%> (-0.59%)	⬇️
...Core/NewRelic.Agent.Core/RuntimeEnvironmentInfo.cs	18.18% <14.28%> (-2.23%)	⬇️

... and 1 file with indirect coverage changes

[chynesNR]

I think RuntimeEnvironmentInfo.DetermineOSPlatform() may need the same protection?

[tippmar-nr]

@chynesNR It does need that protection, but I opted not to add it, as doing so broke the unit tests for that class (we only have .NET 462 unit tests for Core). I'll add a comment to the class that will hopefully dissuade folks from using it outside of the protection of .NET Standard 2.0+.