Use pull_request_target trigger

Author: lee-dohm

.github/workflows/notify-when-maintainers-cannot-edit.yaml

@@ -5,7 +5,7 @@ name: Notify When Maintainers Cannot Edit
 # **Who does it impact**: Open-source.
 
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened