Remove 'group_projects_api_preload_groups' feature flag

What does this MR do and why?

Remove 'group_projects_api_preload_groups' feature flag

The group projects API root ancestor preloader has been in use in production for several months without incident. This removes the feature flag.

• feature flag rollout issue: #354372 (closed)
• typefeature merge request: !81838 (merged)

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