Only run notify_changes job in merge requests [skip modfiles]

Changes introduced

notify_changes job was run on all branches. Here the behaviour is changed so that it only runs on merge requests.

Tests

Before MR creation

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/pipelines/3077445

notify_changes not run

After MR creation

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/16642711

Issues resolved

Closes #