Add 3.4-migration, 3.4 to scala.runtime.stdLibPatches.language

[bishabosha]

We need to ensure that dotty.tools.dotc.config.SourceVersion and scala.runtime.stdLibPatches.language stay in sync, at least with reminder comments, possibly with a test that analyses the tasty.

Compiler version

3.4.0-RC1-bin-SNAPSHOT (982c91b)

Minimized code

import scala.language.`3.4`

println("hello, world")

Output

1 |import scala.language.`3.4`
  |                       ^^^
  |                       value 3.4 is not a member of object language - did you mean language.3.0?

Expectation

compile ok

[bishabosha]

@Kordyjan possibly a test/workflow also that checks that a stable release can't be made without the relevant source version being available