Run conflict labelling action on all pushes

If a conflict is resolved but the action only runs on merge to the select branches then the 'Has Conflicts' label is not removed until a push to this limited set of branches occurs.
Run on all push events to give the most up to date information.

.github/workflows/label_merge_conflicts.yml

 name: 'Check for merge conflicts'
 on:
   push:
-    branches:
-      - master
-      - release-next
+
 jobs:
   label-conflicts:
     runs-on: ubuntu-latest