ci: build docs on Python 3.11 (fastjsonschema dropped 3.9)

The docs job pins Python 3.9, but a transitive dependency
(fastjsonschema) now uses PEP 604 'dict | bool' syntax that is invalid
before 3.10, so sphinx-build fails at startup. Bump to 3.11.13 to match
core_code_checks.yml.

.github/workflows/doc.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v3
         with:
-          python-version: '3.9'
+          python-version: '3.11.13'
       - name: Install dependencies
         run: |
           pip install uv