chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - id: set-matrix
         run: echo "matrix=$(ls challenges/ | jq -R -s -c 'split("\n")[:-1]')" >> $GITHUB_OUTPUT
@@ -33,7 +33,7 @@ jobs:
         challenge: ${{ fromJson(needs.list-challenges.outputs.matrix) }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3