chore: mark scala-library and scalajs-scalalib_2.13 as always instead of semver-spec

[hamzaremmal]

In some scenarios, specifically in cross compilation projects, sbt will complain that scala-library:2.13.x and scala-library:3.x.y might be imcompatible, as they are both declared semver-spec. See en example here:

[error] (update) found version conflict(s) in library dependencies; some are suspected to be binary incompatible:
[error] 
[error] 	* org.scala-lang:scala-library:3.8.0-RC1-bin-SNAPSHOT (semver-spec) is selected over 2.13.0
[error] 	    +- org.scala-lang:scala3-library_3:3.8.0-RC1-bin-SNAPSHOT (depends on 3.8.0-RC1-bin-SNAPSHOT)
[error] 	    +- org.scala-lang.modules:scala-xml_2.13:1.2.0        (depends on 2.13.0)

In this PR, we change the version scheme of both scala-library and scalajs-scalalib_2.13 to always so that the transition between the old stdlib structure and the new one stay invisible to the users.
In the future, when we break binary compatibility (meaning Scala 4), we can revert this PR and have artifacts labeled with semver-spec as it will be exactly that scheme.

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The correctness was validated in #23748 locally.

[skip ci]