Switch to System.Text.Json

[drasticactions]

This pull request makes several changes to allow supporting NativeAOT and trimming.

• Remove Newtonsoft.JSON and replace it with the build-in System.Text.Json
• Use JsonSourceGeneration to handle serialization/deserialization
  Remove the netcore3.0 and net7.0 targets, as they are both unsupported runtimes. Netstandard2.0 should be able to cover those runtimes if someone still uses them.
• Update the tests to reflect switching to System.Text.Json

System.Text.Json makes changes to how it handles encoding Unicode values that would break trying to directly compare the literal strings between the input and output, even though when read back into a JSON Parser, it will read fine. So I changed the tests to compare the objects when reparsed rather than compare the strings, as they would require relaxing encoding checks. Even if the literal strings are not the same, they represent the same values in a safe way that should work cross-platform.