swap trigger (#8463)

.github/workflows/changelog-existence.yml

@@ -2,10 +2,8 @@
 # Checks that a file has been committed under the /.changes directory
 # as a new CHANGELOG entry.  Cannot check for a specific filename as
 # it is dynamically generated by change type and timestamp.
-# This workflow should not require any secrets since it runs for PRs
-# from forked repos.
-# By default, secrets are not passed to workflows running from
-# a forked repo.
+# This workflow runs on pull_request_target because it requires
+# secrets to post comments.
 
 # **why?**
 # Ensure code change gets reflected in the CHANGELOG.
@@ -19,7 +17,7 @@
 name: Check Changelog Entry
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, reopened, labeled, unlabeled, synchronize]
   workflow_dispatch: