Use pull_request_target for close-issues workflow to support fork PRs

The workflow needs access to PHPSTAN_BOT_TOKEN to close linked issues,
which is unavailable for pull_request events from forks.
pull_request_target runs in the base repo context with access to secrets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ondrejmirtes

.github/workflows/close-issues-on-merge.yml

@@ -3,7 +3,7 @@
 name: Close issues from merged PRs
 
 on:
-  pull_request:
+  pull_request_target:
     branches:
       - "2.1.x"
     types: