In #2997 it was pointed out the current branch-trigger matches more
branches than the release-branches. Retrict the match-pattern to only
cover release-branches

Also only trigger builds on the next-branch and not branches prefixed
with this name.

Link: https://github.com/freifunk-gluon/gluon/pull/2997#issuecomment-1771076385



Signed-off-by: David Bauer <mail@david-bauer.net>