chore: update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/CI.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
     - name: Use Node.js
       uses: actions/setup-node@v6
       with: