Replace namespace_id with group_id and project_id for POST /user/runners

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR replaces the namespace_id argument for the POST /user/runners endpoint that was just introduced in this milestone, so that it matches the GraphQL version. My initial idea was that the GraphQL runnerCreate mutation would later leverage ProjectNamespace in order to make everything be a namespace_id, but that will not be practical.

Part of #390427 (closed)

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