Upgrade wagtail support to anything in this major version

[alexm118]

Configures our testing suite to test against the latest release of wagtail instead of a hard set maximum.

Changes

• wagtail < 2.11 -> wagtail < 3

Notes

• This is to align with our new goal of supporting new releases of wagtail based on tests passing, rather than manually confirming the library works.

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
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️