Use shared workflow to publish gem
Use shared workflow to publish gem

This makes use of the shared workflow that has been added to
alphagov/govuk-infrastructure which allows publishing rubygems.

This does remove the yarn install step as that isn't in the shared
workflow, however this isn't needed for publishing the gem.

alphagov/govuk_admin_template,alphagov/govuk_admin_template,alphagov/govuk_admin_template