Fixes https://github.com/scylladb/scylladb/issues/13578
Now that the documentation is versioned, we can remove
the .. versionadded:: and .. versionchanged:: information
(especially that the latter is hard to maintain and now
outdated), as well as the outdated information about
experimental features in very old releases.
This commit removes that information and nothing else.
Closes#13680
