Use pull_request_target for better security with forked PRs (#22024)

.github/workflows/automerge-dependencies.yml

@@ -9,7 +9,7 @@ name: Auto Merge Dependency Updates
 # **Who does it impact**: It helps docs engineering focus on higher value work.
 
 on:
-  pull_request:
+  pull_request_target:
     paths:
       - 'package*.json'
       - 'Gemfile*'