Support Wagtail 3.0

[BrianXu20]

Ticket: https://projects.torchbox.com/projects/support-team/tickets/491

Make the current package support Wagtail 3.0.

I selected some changes from this MR, then added Wagtail 3.0 changes https://github.com/wagtail/wagtail-factories/pull/50

• Drop Python 3.6 (EOL)
• Drop Django 2.2 (EOL), 3.1 (EOL)
• Drop Wagtail 2.11 (EOL), 2.13 (EOL), 2.14 (EOL)
• Drop factory_boy <3.2
• Add Python 3.10
• Add Django 4.0
• Add Wagtail 2.15 LTS and 2.16 and 3.0

[BrianXu20]

Hi, @mvantellingen , @thibaudcolas I wonder if this can get merged and have a release?