chore: drop scala_legacy and MainGenericRunner

[hamzaremmal]

With everything that is happening for 3.8.0, I believe it is now time to remove the old runner infrastructure and the scala_legacy script too. scala-cli has been shipped with every release of Scala since 3.5.0.

In preparation for #24243

[som-snytt]

bin/scalaQ is MainGenericRunner, which I need every time the test rig is broken or too slow. How does everyone else cope?

Should add, avoiding publishLocal. This is for a cycle in a few seconds.

[Gedochao]

@som-snytt it is rather inevitable that we have to do this, as the REPL is being extracted to a separate artifact in #24243.
scala_legacy has been deprecated for a while now, and it's been the plan to have it removed in 3.10.
This now puts it up for discussion whether we should do it now.
I think it may be worth it, so that we have the separate REPL with all its benefits done for 3.9 LTS (plus the necessary tooling changes would happen in 3.8, which requires tooling adjustments for the standard library and other things already).

[Gedochao]

https://github.com/coursier/apps/blob/main/apps/resources/scala3.json
we'll need to update coursier/apps when we do this, BTW.

[hamzaremmal]

And https://github.com/coursier/apps/blob/main/apps/resources/scala3-repl.json too

[Gedochao]

scala_legacy has been deprecated for a while now, and it's been the plan to have it removed in 3.10. This now puts it up for discussion whether we should do it now.

It seems we're doing it in 3.8.0