Switch user destroy to use limited execution time worker

What does this MR do and why?

Starts using Users::MigrateRecordsToGhostUserWorker as part of user deletion behind a feature flag. Users::MigrateRecordsToGhostUserWorker is a new cron worker that migrates and deletes user records in batches with limited execution time running periodically. This helps avoiding long running jobs and transactions.

Extracted from !93702 (closed).

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/366655

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