chore(deps): update docker/setup-buildx-action action to v4

Signed-off-by: milkmaker <milkmaker@mailcow.de>

.github/workflows/rebuild_backup_image.yml

@@ -19,7 +19,7 @@ jobs:
         uses: docker/setup-qemu-action@v4
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
 
       - name: Login to GHCR
         if: github.event_name != 'pull_request'