Use latest-kf5 and stable-kf5 keys in branch-rules.yml

It is unfortunate that you have closed the !407 (closed). But ok, that may be a bit subjective.

Regarding this change, it clearly needs to be done, as people will want to read this file from the point when kde-builder will rely on it.

This changes "@latest" to "@latest-kf5" and "@stable" to "@stable-kf5".

I have prepared the corresponding change for CI-Utilities. See ci-utilities!366.