Upgrade wagtail to 6.0

[josewails]

This PR adds support for wagtail==6.0. It also drops support for Django==3.X.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the development playbook
• Passes all existing automated tests
• Any change in functionality is tested

[dustinblanchard]

@willbarton is it possible to get this reviewed/merged in? This is a blocker for our wagtail 6 upgrade.

[willbarton]

HI @dustinblanchard @josewails. We'll get this committed — there are more changes that are needed (Wagtail has updated the breadcrumb pattern in the admin again, which breaks navigability, but we might fix that separately).

I do want to open up Wagtail > 5.2 support though without an upper pin while we fix that, so this isn't quite going to be the right change here.

I'm sorry it's taken so long!

[willbarton]

I've opened #45 which should unblock Wagtail 6.x upgrades in exchange for a slightly broken treemodeladmin until we can fix the UI.

When we get the UI fixed, I'll bring these commits over the to bump the version numbers, etc.