Specification should be build from stable releases

[WojciechMazur]

Currently specification is build on each commit to main and published to https://scala-lang.org/files/archive/spec/3.4/ even though it might contain data specific to Scala 3.5+ or nightly versions.

• Specification should be build only from stable release, similary as it is done to reference docs:
  • we can change CI to publish it only from language-referecne-stable branch
• On new minor release the config should be adjusted to produce new version of the spec, eg. https://scala-lang.org/files/archive/spec/3.8/
• Old versions of the spec should be available as an achive (currently accessible by link, in the future we can add version select menu)