Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
       - uses: pre-commit/action@v3.0.1
 
@@ -34,7 +34,7 @@ jobs:
           - mypy
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
           python-version-file: pyproject.toml