Upgrade to Python 3.12

[sergei-maertens]

This is now possible as we upgraded to Django 4.2.

Python 3.11 offers some performance improvements.

Checklist (WIP):

• https://github.com/open-formulieren/open-forms/pull/3974 (3.12 removes setuptools). See the PR description for more details on what's needed.
• [ ] Run pyupgrade with Ruff
• [ ] Bump dependencies that requires building (no wheels provided, current pinned versions fails to build on 3.12): ruamel-yaml, ruamel-yaml-clib, greenlet, lxml, jq
• [ ] Switch from psycopg2 to psycopg (3). psycopg2 has no wheels and build fails on 3.12.