Finalize the finish onboarding for enterprise user backfill

What does this MR do and why?

Finalize the finish onboarding for enterprise user backfill

• this only ever does work on Saas instances due to the saas feature guards around users.onboarding_in_progress the the BackfillFinishOnboardingForEnterpriseUser class uses in the query. That is guarded in https://gitlab.com/gitlab-org/gitlab/blob/3e1bc33653f7f62fcdcc5a61ea54e35097ef805d/ee/app/services/onboarding/status_create_service.rb#L15-15.

• We can, I believe, skip the required stop here as defined in https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration as self-managed hosts are not a concern here. Please check me on this.

• The current query in postgress AI shows there are no more items that are out of compliance with the work the BBM was meant to do: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/38358/commands/117673

• status: finished ✅

Changelog: other

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Related to #520498 (closed)