branching: create a merge request from branch-update-infrastructure instead of...

branching: create a merge request from branch-update-infrastructure instead of direcly pushing to main

This job push a commit to the repo where the branching pipeline is
currently running. Consequently, the actual branching pipeline
is cancel by a new dummy one based on the new commit. To avoid
cancelling the current pipeline, instead we create a merge request
that can be merged once the branching pipeline is completed to
avoid cancellation.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>