Add support for Django 6.0

Currently Renovate wants to upgrade our Django in this repo to v6 however it is currently unaware of the python version needed for this in copier.yml and .gitlabci.yml.

Decide how we want to upgrade our Django and Python versions in this project and teach renovate how to update them.

At writing this issue Django 6.0 is the latest Django but 5.2 is the current LTS version. Django 6.2 will be the next LTS release and will be released in April 2027. Django 6.0 will be supported until April 2027.

For full detains see Django versions

Python 3.14 is the latest full release of python (3.15 is in development). There are 3.14.x releases of python as bug fixes etc

At least Python 3.12 is needed for Django 6.0