Adds Regulation publication subtype

Bumps govuk_content_models gem to version 20.1.0 so that Panopticon can handle the new publication subtype.

In support of story https://www.pivotaltracker.com/story/show/78637270
I think this should be fine -- this breaking change modified how Legislative Lists were rendered into HTML which (as far as I know) Panopticon isn't responsible for, it's the publishing apps themselves (`specialist-publisher` and `whitehall`) which have already had data migrations run on them to ensure govspeak is in the correct format.

OTOH if I'm wrong and Panopticon *does* handle converting govspeak to HTML, and that govspeak includes LegislativeLists, then this **is** an Issue™.