Bump govuk_content_models to 22.2.0

This version bump includes the RAIB Report artefact kind and a breaking
change that removes safe_html validation in non-govspeak fields. This
should be fine, as Rails takes care of these fields anyways.
Merge pull request #228 from alphagov/bump-govuk-content-models-to-22-2-0

Bump govuk_content_models to 22.2.0