Trigger on auto_merge_enabled

The trimming needs to be performed before the PR is merged so that the commit
message is based on the trimmed version. This prevents us from using events
such as the PR getting closed or merged. Triggering on merge queue events is
also problematic, because a merge_queue event doesn't have the equivalent of
"pull_request.user" and we need that to test if the PR came from Dependabot.
So instead, this workflow triggers when auto-merge is enabled for the PR.

Author: mhucka

.github/workflows/dependabot-pr-trimmer.yaml

@@ -13,16 +13,23 @@
 # limitations under the License.
 
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-# Remove bloat from Dependabot's PR messages
+# Trim parts of the Dependabot PR text that are unnecessary in the git history.
+#
+# The trimming needs to be performed before the PR is merged so that the commit
+# message is based on the trimmed version. This prevents us from using events
+# such as the PR getting closed or merged. Triggering on merge queue events is
+# also problematic, because a merge_queue event doesn't have the equivalent of
+# "pull_request.user" and we need that to test if the PR came from Dependabot.
+# So instead, this workflow triggers when auto-merge is enabled for the PR.
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 name: Dependabot PR trimmer
 run-name: Filter message body of PR ${{github.event.pull_request.number}}
 
 on:
-  merge_group:
+  pull_request:
     types:
-      - checks_requested
+      - auto_merge_enabled
 
   workflow_dispatch:
     inputs: