ci: fetch full history in the merge-up job

The "Create Merge-Up Pull Request" job checked out with the default shallow
fetch, so the source release branch (e.g. 11.4.x) was not available locally and
laminas/automatic-releases failed with "fatal: not a valid object name: '11.4.x'".
Use fetch-depth: 0 (as the bump job already does) so all branches are present.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Spomky

.github/workflows/release-on-milestone-closed.yml

@@ -34,6 +34,8 @@ jobs:
     steps:
       - name: "Checkout"
         uses: "actions/checkout@v6.0.2"
+        with:
+          fetch-depth: 0
 
       - name: "Create Merge-Up Pull Request"
         uses: "laminas/automatic-releases@1.26.2"