Update Product Planning pages' metadata

doc/administration/reply_by_email.md

 ---
 stage: Plan
-group: Certify
+group: Product Planning
 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
 ---
 

doc/api/epic_issues.md

 ---
 stage: Plan
-group: Portfolio Management
+group: Product Planning
 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
 ---
 

doc/api/epics.md

 ---
 stage: Plan
-group: Portfolio Management
+group: Product Planning
 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
 ---
 

doc/user/group/epics/index.md

 ---
 type: reference, howto
 stage: Plan
-group: Portfolio Management
+group: Product Planning
 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
 ---
 

doc/user/group/epics/manage_epics.md

 ---
 type: howto
 stage: Plan
-group: Portfolio Management
+group: Product Planning
 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
 ---
 

doc/user/group/roadmap/index.md

 ---
 type: reference
 stage: Plan
-group: Portfolio Management
+group: Product Planning
 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
 ---
 

doc/user/project/requirements/index.md

 ---
 type: reference, howto
 stage: Plan
-group: Certify
+group: Product Planning
 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
 ---
 

doc/user/project/service_desk.md

 ---
 stage: Plan
-group: Certify
+group: Product Planning
 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
 ---