page.role not assigned since document attribute docrole was renamed to role

When Antora migrated to Asciidoctor.js 2, the page.role assignment in the UI model broke. In Asciidoctor 2, the inheritance of the role attribute was removed. That meant that role for the document no longer needed to be stored under a unique name and thus the docrole attribute was renamed back to role. The page composer in Antora was not updated accordingly. Therefore, the page.role variable is no longer picking up the role from the AsciiDoc document.