Merge stable into devel whenever stable is updated, rather than via a schedule
Merging via a schedule could not happen regularly (if at all): any time the master→stable CI pipeline completes before the stable→devel one, new commits are pushed to stable, which "aborted the automatic merge because the source branch was updated", and then the setup will not recover automatically without human action, as we've seen on !1899 (merged).
So let's instead merge as part of the regular, non-scheduled pipeline, whenever the stable branch is updated. Incidentally, this is exactly the semantics we want.
Credits: @zen on !1889 (comment 251843)
Edited by intrigeri