Upgrade django to 5.2, python to 3.13

[daveoconnor]

This includes the following changes, related to ticket #1911:

• Upgrades Django to 5.2 with code updates.
• Upgrades Python to 3.13.
• Adds django-upgrade, as a just command and a pre-commit hook.
• Adds documentation for dependency upgrade process.
• Removes unused (and unmaintained) django-rest-auth.
• Updates github workflows to Python 3.13.

I'm aiming for the 4.2 upgrade to go to prod and then we test this locally for a few days.

[daveoconnor]

HI @sdarwin, CI actions are failing because server versions of PostgreSQL are <14 (v12).

For local development we've been using v16 in the docker-compose.yml, most recent PostgreSQL is v17, with v18 coming on the 25th of September.

This isn't very urgent, doesn't block Django 4.2 we want out sooner.

[sdarwin]

@sdarwin, CI actions are failing because server versions of PostgreSQL are <14 (v12).

Then in this PR adjust the postgres version (v12) in .github/workflows/actions.yml and .github/workflows/actions-gcp.yaml

The server itself is v16. We should plan to stay on that, upgrading at the same time as the server, which is approximately every 2 yrs with Ubuntu upgrades.

[daveoconnor]

Thanks, was wary of changing those in case that caused issues elsewhere. Done now.

[gregjkal]

LGTM! One small question.

[karimarie67]

@daveoconnor We discussed this one a while back "that one should be done later, can wait for now. Probably should in fact, would be a couple of weeks before that's going out so should get updated in the meantime". Can this one go now? Thank you!

[daveoconnor]

I'd like to merge this one after a merge to master so we can use it locally for close to a week. If you've QAd it I can merge it after tomorrow's merge.