[CI] Handle unlabeled events in on-merge workflow (#194058)

## Summary

Closes #194051

As far as I can tell from the docs the `unlabeled` event sends
information about the removed label. I'm not sure the best way to test
this aside from merging the PR and adjusting the labels on this PR
afterwards. Reverting if needed.

Plan:
1. Merge PR
2. Add `backport:prev-minor`
3. Verify the workflow ran and _did not_ trigger a backport since
`backport:skip` is still a label
4. Remove `backport:skip`
5. Verify the workflow ran and _did_ trigger a backport to `8.x`
6. Revert the labels and close the backport PR since it is not actually
needed.

.github/workflows/on-merge.yml

@@ -3,6 +3,7 @@ on:
     types:
       - closed
       - labeled
+      - unlabeled
 
 jobs:
   on-merge:
@@ -21,6 +22,7 @@ jobs:
             || github.event.label.name == 'auto-backport'
           )
         )
+        || (github.event.action == 'unlabeled' && github.event.label.name == 'backport:skip')
         || (github.event.action == 'closed')
       )
     steps: