Drop Scala 2 in config and specification-level modules & bump jsoniter-scala to 2.38.2 (was 2.13.5.2)

[Gedochao]

Following up on #3911

• the config module will no longer be cross-compiled for Scala 2, nor will it target Java 8 (cc @alexarchambault)
• similarly, specification-level no longer needs Scala 2 cross-compilation
• removed some Scala-2-specific hacks which are no longer needed as a result
• bumped jsoniter-scala from an antique 2.13.5.2 version (which we kept cuz it still supported Java 8) to 2.38.2
  • this actually fixes the JSON export to respect the format we have in our docs (I adjusted the tests, which assumed a different format)

Requires

• Drop Scala 2 in runner, test-runner and tasty-lib modules #3911