Remove support for versions of Wagtail < 4.1 (Wagtail 4.1 or later now required)
Note: It is important that this PR be merged first (https://github.com/nhsuk/wagtail-nhsuk-frontend/pull/243) as it provides support for Wagtail 4.0 while this current PR removes support for versions of Wagtail older than 4.1
Note: It is important that this PR be merged first (https://github.com/nhsuk/wagtail-nhsuk-frontend/pull/243) as it provides support for Wagtail 4.0 while this current PR removes support for versions of Wagtail older than 4.1