Tests | Update Package Versions

[benrr101]

Recreation of #2980 to rebase it onto main, the original pull request description is below

---

• Upgrade packages only used in test projects to latest versions
• Upgrade XUnit / MSTest SDK
• Remove unused variables
  • MicrosoftNETCoreRuntimeCoreCLRVersion
  • MicrosoftDotNetPlatformAbstractionsVersion
  • SystemTextEncodingCodePagesVersion
• Remove reference to xunit.runner.utility, this should not be linked as per xunit.runner.visualstudio 2.8.1 fails to discover .Net 4.7.2 Unit Tests xunit/visualstudio.xunit#409 (comment)

According to #2379 (comment), the Remote Executor package and XUnit was held back by .NET6, now that .NET6 is removed, this should work fine

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.85%. Comparing base (98b2c84) to head (ce75cb5).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3312       +/-   ##
===========================================
- Coverage   75.43%   64.85%   -10.58%     
===========================================
  Files         262      298       +36     
  Lines       41182    65537    +24355     
===========================================
+ Hits        31065    42505    +11440     
- Misses      10117    23032    +12915     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	68.21% <ø> (-7.10%)	⬇️
netfx	65.99% <ø> (?)

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