Merge Portfolio Management and Certify under Product Planning

Issue to track merging ~"group::portfolio management" and groupcertify into a new group "Product Planning"

Need to Update

• Rename ~"group::portfolio management" to groupproduct planning
• Bulk update groupcertify issues/epics/MRs to groupproduct planning, delete groupcertify
• Update Product Categories page: https://about.gitlab.com/handbook/product/product-categories/#plan-stage
• Update Plan Stage team page https://about.gitlab.com/handbook/product/product-categories/plan/
• Update Plan Stage direction page https://about.gitlab.com/direction/plan/
• Update engineering team pages https://about.gitlab.com/handbook/engineering/development/dev/plan-portfolio-management-be/, https://about.gitlab.com/handbook/engineering/development/dev/plan-certify-be/
• Update team build board with new label and link from the engineering team pages
• Update engineering dashboards and triage automation gitlab-org/quality/triage-ops#578 (closed)

Discuss w/ Marketing (some pages on about.gitlab.com discuss the Portfolio Management capability/features not necessarily the internal "group") cc @cfoster3

• https://about.gitlab.com/solutions/portfolio-management/