Rename migration-related navigation items

What does this MR do and why?

Related to: gitlab!109443 (merged), updates the names of some ~"group::import" pages on the global navigation.

Screenshots, screen recordings, or links to review app

Preview (see navigation items):

• https://eread-rename-migration-related-navigation-items.docs.gitlab-review.app/ee/development/bulk_import.html
• https://eread-rename-migration-related-navigation-items.docs.gitlab-review.app/ee/api/bulk_imports.html

Merge request acceptance checklist

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