Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -26,7 +26,7 @@ jobs:
           - mypy
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
           python-version: "3.14"