bug-1958216: upgrade Django to 5.2.11

[biancadanforth]

Because:

• Django 4.2 LTS is hitting EOL in April 2026.
• We need to upgrade to the next LTS, which is Django 5.2.
• See upgrade plan for more information.

This PR:

• Upgrades Django in a stepwise fashion from 4.2 -> 5.0 -> 5.1 -> 5.2, checking for test failures at each step.
• Fixes any test-related issues -- there were no actual test failures at any upgrade step -- only filter warnings to adjust.

Note: I also ran django-upgrade with a target version of 5.0 as part of working on the first upgrade step from 4.2 to 5.0, but no upgrade issues were found.

[smarnach]

Apart from the Django version number, this looks good to me.

When verifying this locally, I noticed the Google dependencies are somehow messed up, and some of the transitive dependencies don't meet the requirements of the packages depending on them. I have a fix for this locally, but I'd like to get this PR merged before applying it. Once this PR and the dependency resolution fix are merged, I can do the uv migration.