Wagtail 4.0 -> 5.1 upgrade + 5.2

[thibaudcolas]

Current on 4.0.3. We’d want to upgrade to 5.1. Ideally to 5.2 if wagtail-localise compatibility makes this possible.

• Also including Python upgrade (3.12 if possible, if not any other)
• And Django (5.0 beta 1 if possible, if not any other)

[Tijani-Dia]

I've linked PRs for the Wagtail, Django and Python upgrades.

wagtail-localize restricts us to use Wagtail 5.0. Looking at the Compatible Django / Python versions on the Wagtail docs, I've aimed for Django 4.2 and Python 3.11.