GH Actions/label mngmnt: add additional triggers for merge conflict check

... in an attempt to get the action runner to remove the "has conflict" label when a PR is updated (which hopefully would have resolved the conflict).

Author: jrfnl

.github/workflows/label-merge-conflicts.yml

@@ -1,9 +1,17 @@
 name: Check PRs for merge conflicts
 
 on:
+  # Check for new conflicts due to merges.
   push:
     branches:
       - master
+  # Check conflicts in new PRs and for resolved conflicts due to an open PR being updated.
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+      - reopened
 
 # Cancels all previous workflow runs for the same branch that have not yet completed.
 concurrency: