[3.12] gh-121277: Allow `.. versionadded:: next` in docs (GH-121278) (GH-125980)

Make `versionchanged:: next`` expand to current (unreleased) version.

When a new CPython release is cut, the release manager will replace
all such occurences of "next" with the just-released version.
(See the issue for release-tools and devguide PRs.)

(cherry picked from commit 7d24ea9db3e8fdca52058629c9ba577aba3d8e5c)

Also backports a minor fix-up:
gh-121277: Raise nice error on `next` as second argument to deprecated-removed (GH-124623)

(cherry-picked from commit e349f73a5ad2856b0a7cbe4aef7cc081c7aed777)

Co-authored-by: Petr Viktorin <encukou@gmail.com>