Reference `-source` versions list should be updated

[WojciechMazur]

https://docs.scala-lang.org/scala3/reference/language-versions/source-compatibility.html

The -source setting documentation is severely outdated:

• future versions should not be recommended as migration from LTS. This mode should be rather discouraged and well described as a very long term goal on how Scala syntax might look like if we'd break Scala 2 compat
• The -rewrite behaviours introduced after 3.3 should be collected and described in here
• Potentially we should create a table describing what syntax was added / deprecated / removed or provides automatic rewrites under given -source versions

It should be finalised before 2nd LTS as it would be really needed for migrations.

Motivation: large number of -source:future or -source:future-migration settings used in OpenCB projects that are in fact not future-ready but rather mostly-3.5-ready or featured in the blogposts and recommended to other users.

[WojciechMazur]

Soft assigning myself for this task, but I might not have time for that for the next few weeks. If anybody else would like to work on it fell free to assign yourself.

[sjrd]

I disagree about -source:future. I wouldn't discourage people from using it. In a greenfield project that doesn't need Scala 2 compat, it's a very reasonable way to develop a project. However, it comes with way fewer guarantees of no source breakage during upgrades. As a consequence, a project using -source:future shouldn't qualify for the open community build.

[Gedochao]

As a consequence, a project using -source:future shouldn't qualify for the open community build.

I wonder about that part... I'd like us to be able to track when we break such projects.
Regressions with -source:future can be treated with lower priority, but perhaps it's still good to know when we cause them.