Add migration to delete orphaned security policy bot users

Andy Schoenenrequested to merge
554741-add-background-migration-to-cleanup-orphaned-security-policy-bots into master

What does this MR do and why?

Security policy bot users can become orphaned when projects are deleted, leaving unused database records. This batched background migration safely removes these orphaned bots that have no project memberships or ongoing ghost user migrations to clean up the database.

We have added a cron worker to regularly delete those users in !196257 (merged) This migration should take care of the initial cleanup

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How to set up and validate locally

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Related to #554741 (closed)

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