Build Scala CLI with Scala 3 Next

[Gedochao]

• builds Scala CLI with Scala 3 Next, so currently 3.9.0 (well, most of it - see below)
• pins the config module at 3.9.0, as it's used by other tools as a dep (i.e. coursier)
• pins the specification-level module at 3.3 LTS until its EOL
• similarly, runner and test-runner stay at 3.3 LTS until its EOL
• all modules at 3.3 LTS will now be built with -Yfuture-lazy-vals and -java-output-version:17 settings
• sources were migrated to newer syntax, where applicable (mostly compiler rewrites, and not a lot of it)

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run)

How much have your relied on LLM-based tools in this contribution?

Yes, although far less than I initially assumed I would 😅

How was the solution tested?

existing tests should suffice