Add tier badge to email notification page

doc/user/profile/unknown_sign_in_notification.md

 ---
-type: concepts, howto
 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
 ---
 
-# Email notification for unknown sign-ins
+# Email notification for unknown sign-ins **(FREE)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/27211) in GitLab 13.0.