Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/reuse.yaml

@@ -9,6 +9,6 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: REUSE Compliance Check
         uses: fsfe/reuse-action@v6