[5.1.x] Upgraded to Python 3.12, Ubuntu 24.04, and enabled fail_on_warning for docs builds.

Backport of 73d532d9a9 from main.

.github/workflows/docs.yml

@@ -21,8 +21,7 @@ permissions:
 
 jobs:
   docs:
-    # OS must be the same as on djangoproject.com.
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     name: docs
     steps:
       - name: Checkout