Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         python-version: ["3.11"]
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with: