Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/python-app.yml

@@ -27,7 +27,7 @@ jobs:
           - 5432:5432
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Install uv
       uses: astral-sh/setup-uv@v7
       with: