Update govuk_content_models to 7.1.0

This gem in this app needs to be kept in lockstep with [govuk_content_api](https://github.com/alphagov/govuk_content_api/blob/2b32d12f2402b82209018633202273f1fa4934a9/Gemfile#L12) and [panopticon](https://github.com/alphagov/panopticon/blob/81a432b8af86612d183808bc7d3e65cb6fe5cc0d/Gemfile#L41) or else the hydration process will break.