Try pull_request_target instead of pull_request (#19081)

A GitHub action to auto-add milestones were added in #18935

This check has been failing consistently with a 403 error on all PRs coming from forks once they are merged into master. Given the 403, it does indicate a permissions issue likely with the GitHub token used for this. Trying pull_request_target instead of pull_request similar to the existing labeler action which uses the same. Since this specific add-milestone check doesn't export any credentials, I believe using pull_request_target should be okay and should fix the problem.

Author: abhishekrb19

.github/workflows/pr-merged.yml

@@ -20,7 +20,7 @@
 name: "PR Merged"
 
 on:
-  pull_request:
+  pull_request_target:
     types: [closed]
 
 jobs: