Update checkout and setup-python to latest versions

Fix warnings about deprecated Node.js 16 actions.

See:

- #1493
- https://github.com/Kozea/Radicale/actions/runs/8943158465/attempts/1

.github/workflows/generate-documentation.yml

@@ -8,7 +8,7 @@ jobs:
   generate:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           ref: gh-pages
       - name: Run generator