[TASK] Migrate backport to shared reusable workflow

Replace the inline backport workflow with a call to the shared
reusable workflow in TYPO3-Documentation/t3docs-ci-deploy.

Reduces 17 lines of per-repo configuration to 4 lines,
providing a single maintenance point for action versions
and consistent behavior across all repos.

Author: CybotTM

.github/workflows/backport.yml

@@ -6,17 +6,6 @@ on:
 
 jobs:
   backport:
-    if: github.event.pull_request.merged == true
-    runs-on: ubuntu-latest
-
-    permissions:
-      contents: write
-      pull-requests: write
-
-    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v 6.0.2
-
-      - name: Backport
-        uses: korthout/backport-action@4aaf0e03a94ff0a619c9a511b61aeb42adea5b02 # v4.2.0
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
+    uses: TYPO3-Documentation/t3docs-ci-deploy/.github/workflows/reusable-backport.yml@main
+    with:
+      label-pattern: "backport *"