Correct metadata of external auth doc

doc/user/admin_area/settings/external_authorization.md

 ---
-stage: none
-group: unassigned
+stage: Manage
+group: Access
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
-type: reference
 ---
 
 # External authorization control **(FREE SELF)**