Update to Django 4.2

[jonwzheng]

The Django version used by RMG-website is no longer supported. We should update Django to version 4.2 LTS (or, 3.2 and then 4.2) to avoid any conflicts down the road.

https://www.djangoproject.com/download/#supported-versions

[jonwzheng]

Proposed roadmap:

Immediate future

1. Enforce Django=2.2.17, migrate to Python version 3.9, and resolve dependency issues (if any)
2. When the Python 3.9 change is merged in, do not install Julia
3. Update the environment file (removing overlapping environment dependencies and Julia)
4. Ensure that website can be built and debugged locally with Python 3.9 + RMG minus julia.
5. Host the new website server with this set-up, with Django 2.2.
6. Upgrade to Django 3.2 and then Django 4.2 LTS. (consider https://pypi.org/project/django-upgrade/ and see online for other resources.)
7. Finally, update the website server with the new changes.

In the future

1. implement CI to make sure the environments can build properly. OR dockerize
2. Implement cron jobs to automatically build RMG-Py and RMG-website.