Wagtail 4.0 upgrade

[bcdickinson]

This PR upgrades the project and it's dependencies (as far as possible) to be compatible with Wagtail 4.0.

To make life easier for myself in terms of being able to easily detect incompatible versions and improve build reproducability, I migrated from pip/requirements.txt to poetry==1.2.1 for dependency management first.

I hit the following snags while upgrading the various wagtail* 3rd party packages:

• wagtail-ab-testing explicitly does not support wagtail>=3 so poetry refuses to install it. The are open PRs to add support (hat tip to @nickmoreton), but they didn't make it in in time for this PR, so for the time being I've just removed A/B testing from this project.
• wagtail-airtable has some incompatibiliites with Wagtail 4.0 so the sync currently doesn't work and should not be used. I've elected not to remove it at this point to avoid losing the airtable_record_id data on AirtableMixin subclasses.
• wagtail-content-import, wagtail-image-import, and wagtailaltgenerator all superficially appear to be compatible with Wagtail 4.0 with the caveat that I haven't done extensive manual testing and the packages themselves don't officially support or test against it.
• wagtail-transfer doesn't officially support or explicitly test against Wagtail 4.0, but the nightly builds against main are currently passing, so we're on pretty safe ground here
• wagtailmedia is fine, it explicitly supports and tests against Wagtail 4.0

I've raised tickets to reinstate wagtail-ab-testing following it being brought up to date and to add 4.0 support to wagtail-airtable.

[zerolab]

The vast majority of changes make sense 👍

Left a number of comments/questions