Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - run: sudo apt-get install -y --no-install-recommends git-crypt