Support Scala 3 -Yscala-output-version flag

[lolgab]

The new -Yscala-release requires special build tool support.

This is a problem Sbt is also facing: sbt/sbt#6814
Scala community discussion: https://contributors.scala-lang.org/t/improving-scala-3-forward-compatibility/5298/22

Possible solution in ScalaModule (inspired by the discussion in the Sbt Issue):

def scalaOutputVersion: T[String] = T { scalaVersion() }
def scalaVersion: T[String]
def mandatoryScalacOptions: T[Seq[String]] = T {
  if(scalaBinaryVersion(scalaVersion()) != scalaBinaryVersion(scalaOutputVersion())) Seq("-Yscala-release", scalaBinaryVersion(scalaOutputVersion()))
  else Seq.empty[String]
}

[lefou]

Sounds reasonable to me.

[prolativ]

The flag will be renamed to -scala-output-version starting from 3.1.2-RC2.
I seem to have a working implementation #1764 but it needs to wait for the release of 3.1.2-RC2 and I would like to align it with the implementation for sbt to make sure the behaviour of the setting are possibly the same no matter what build tool a user chooses but it seems not so easy to achieve exactly the same semantics in sbt regarding which version of the standard library gets used at runtime.

[lolgab]

Since the compiler team decided to drop the -scala-output-version flag we don't need to support it anymore.
scala/scala3#15146