.github: upgrade Python to 3.12

Author: jjokella

.github/workflows/docs.yml

@@ -27,10 +27,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python 3.11
+      - name: Set up Python 3.12
         uses: actions/setup-python@v5
         with:
-          python-version: 3.11
+          python-version: 3.12
           cache: 'pip'
 
       - name: Install dependencies