Problem

Django 5 is out and the initial bugs are sorted. It has some significant QoL and other improvements for async routes, and we should update to Django 5.

Description

Review the Django 5 release notes and cross-reference breaking changes with our API codebase.

https://docs.djangoproject.com/en/5.0/releases/5.0/#backwards-incompatible-changes-in-5-0

At a glance, I don't think we are affected by the breaking changes, but please take a close look at each item to confirm this.

Update the API to Django 5 and make any necessary changes.

Remove any backported code that may exist to add Django 5 features before we've upgraded (e.g., cache_control backport for #4005).