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

Author: renovate[bot]

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       CI: true
     steps:
     # Setup.
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Use Node.js ${{ matrix.node-version }}
       uses: actions/setup-node@v4
       with: