Switch to Scala 3-based case-app

[alexarchambault]

This makes us use the Scala 3 artifacts of case-app (including the macros deriving CLI options parsers).

This is handled in 3 steps here:

• first we use the Scala 3 artifacts from the cli module only (not from cli-options that derives parsers), and adjust some API calls - this breaks things at runtime (because of case-app Scala 2 and Scala 3 artifacts compatibility issues)
• then we use them from cli-options too
• lastly, we update to a version of scala-cli-signing relying on a Scala 3 case-app, so that there's no runtime issues with the scala-cli-signing stuff either.

The changes of the first two points mainly revolve around dropping the uses of Parser.Aux stuff (required to make the Scala 2 typer happy, and not necessary anymore in Scala 3).