Fix auto-merge: use pull_request_target to get workflows:write permission

Dependabot-triggered pull_request events get read-only GITHUB_TOKEN
regardless of the permissions block, so workflows:write had no effect.
Using pull_request_target runs in base-branch context with full permissions,
which allows merging PRs that modify workflow files.

Author: woutervh

.github/workflows/dependabot-auto-merge.yaml

@@ -9,7 +9,7 @@
 name: Auto Merge Dependabot
 
 on:
-  pull_request:
+  pull_request_target:
 
 permissions:
   contents: write