Upgrade Django to latest LTS release

[jaguillette]

According to the release schedule outlined here: https://www.djangoproject.com/download/, we should stick with the latest Django 3.2 until 4.2 is released, then stick with that until there's a 5.2 a few years later. 4.1 is in progress, and 4.2 is expected to release at some point in 2023, at which time we can start thinking about switching to it.

For the purposes of this story, updating Django means updating to the latest version of 3.2.