chore: add GitHub Action to automatically close PRs on subtree splits (#6648)

Author: dunglas

.github/workflows/close_pr.yml

@@ -0,0 +1,13 @@
+name: Close Pull Request
+
+on:
+    pull_request_target:
+        types: [opened]
+
+jobs:
+    run:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: superbrothers/close-pull-request@v3
+              with:
+                  comment: "Thank you for your pull request. However, you have submitted this PR on a read-only sub split of `api-platform/core`. Please submit your PR on the https://github.com/api-platform/core repository.<br><br>Thanks!"