v2 - Drop old stuff, require .NET 6+

[natemcmaster]

Breaking changes!

• Dropping support for directly parsing the deps.json file. This is usually not required anymore. Since .NET Core 3, the built-in API AssemblyDependencyResolver handles those files.
• Dropping support for .NET Core 2.1 and 3.1 in the NuGet package.

Other cleanups

• Cleanup dead code
• Simplify complication flags now that the project doesn't need to cross-target
• Update test dependencies to latest versions

[codecov[bot]]

Codecov Report

Merging #252 (5e6fdb8) into main (452f8d3) will decrease coverage by 16.47%. The diff coverage is 63.63%.

:exclamation: Current head 5e6fdb8 differs from pull request most recent head 2873b0c. Consider uploading reports for the commit 2873b0c to get more accurate results

@@             Coverage Diff             @@
##             main     #252       +/-   ##
===========================================
- Coverage   82.87%   66.39%   -16.48%     
===========================================
  Files          15       14        -1     
  Lines         578      494       -84     
===========================================
- Hits          479      328      -151     
- Misses         99      166       +67     

Impacted Files	Coverage Δ
src/Plugins/Loader/AssemblyLoadContextBuilder.cs	65.30% <ø> (-17.22%)	:arrow_down:
src/Plugins/Loader/ManagedLoadContext.cs	63.38% <ø> (-15.98%)	:arrow_down:
src/Plugins/PluginConfig.cs	84.21% <ø> (ø)
src/Plugins/PluginReloadedEventHandler.cs	0.00% <ø> (ø)
src/Plugins/PluginLoader.cs	79.13% <55.55%> (-2.12%)	:arrow_down:
src/Plugins/Loader/RuntimeConfigExtensions.cs	71.11% <100.00%> (-22.23%)	:arrow_down:
src/Plugins/LibraryModel/NativeLibrary.cs	0.00% <0.00%> (-100.00%)	:arrow_down:
src/Plugins/LibraryModel/ManagedLibrary.cs	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 2 more

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