The package should set `default_auto_field`

[th3hamm0r]

Our project sets DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField' in Django's settings.
This leads to a new migration being created automatically in the (thirdparty) wagtail_ai-package, since it currently uses AutoField.

I think the apps.py should set default_auto_field = "django.db.models.AutoField" to avoid a migration being created.

This is equal to how it has been fixed for wagtailmedia for example: torchbox/wagtailmedia#134