v4.1.0: Wagtail 5.0 support

This release adds support for Wagtail 5.0, and drops support for Wagtail < 4.1.

The upper bound on the Wagtail version was also removed to ease the upgrade path in future.

What's Changed

• Add Wagtail 5.0 support, drop Wagtail < 4.1 support by @jams2 and @nickmoreton in #74

Full Changelog: 4.0.0...v4.1.0