Build System - Use Ubuntu 22.04 for GitHub actions docs build.

If we use Ubuntu 24.04 (the same as ubuntu-latest), the build will fail
because Python 3.7 is no longer available on Ubuntu 24.04.

Author: david-cattermole

.github/workflows/build_and_deploy_docs.yml

@@ -10,7 +10,9 @@ permissions:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # Ubuntu 24.04 does not have Python 3.7 anymore, so we must move
+    # back to Ubuntu 22.04 for compatibility.
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         python-version: ["3.7"]