Add support for Wagtail 6.0

[chosak]

This change adds support for Wagtail 6.0 and augments the testing matrix with a combination of Wagtail 6 + Django 5.

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

[wpears]

Looks cool, can you also update the Compatibility section in the README?

[chosak]

Nice catch @wpears, done in d9b0695.