Support/wagtail42

[nickmoreton]

This updates the package top support Wagtail>=4.1

and removes compatibility support for Wagtail versions < 4.1

Support ticket: https://torchbox.monday.com/boards/1124794299/pulses/1124794780

[easherma]

Plenty of folks would still be using wagtail versions < 4.1. Would this make the library unusable in those cases?

[zerolab]

Plenty of folks would still be using wagtail versions < 4.1. Would this make the library unusable in those cases?

@easherma it would make the next release unusable. All current versions will continue to work.

[cnk]

Does this PR still make sense given this item in the Wagtail 5.0 Release notes:

• Include wagtail-factories in wagtail.test.utils to avoid cross-dependency issues (Matt Westcott)

[zerolab]

Superceded by #74