Reconfigure Product Planning categories to industry capabilities

Why is this change being made?

Product Planning currently has 3 categories:

• Epics
• Roadmaps
• Design Management

"Design Management" is an example of a good category since it's a "high-level capabilities that may be a standalone product at another company". That page also goes on to mention that "we should map categories to vendor categories defined by analysts" (even citing Portfolio Management as an example, which we moved away from as a group name). Based on this definition, Epics and Roadmaps should be improved as categories since they're not high-level capabilities.

Instead, this MR proposes that we reconfigure these categories as:

• Program Management
• Portfolio Management
• Design Management

Epics (Program Management) and Roadmaps (Portfolio Management) would still fall under this group, but these are industry terms/categories that are recognizable, searched for, and recognized by analysts.

Original issue: https://gitlab.com/gitlab-org/plan/-/issues/210

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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