Update Custom Project Templates stage metadata

doc/user/admin_area/custom_project_templates.md

 ---
-stage: Plan
-group: Project Management
+stage: Manage
+group: Import
 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/#designated-technical-writers
 type: reference
 ---