Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/backend.yml

@@ -31,7 +31,7 @@ jobs:
           ACCESS_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6
 
       - name: Setup php extensions cache
         id: extcache