chore: Replace pull_request_target with pull_request in PR title validation (#871)

Replace `pull_request_target` with `pull_request` to reduce attack
surface. The semantic PR title action only reads the event payload, so
elevated permissions are not needed.

Author: erezrokah

.github/workflows/pr_title.yml

@@ -2,7 +2,7 @@
 name: "Validate PR title"
 
 on:
-  pull_request_target:
+  pull_request:
     types:
       - opened
       - edited