Create migration to cleanup orphaned security policy bots

We want to run a batched background migration to clean up existing orphaned security policy bots before enabling the security_policy_bot_cleanup_cron_worker feature flag.

!196257 (merged) added a new cron worker to automatically cleanup orphaned security policy bots going forward. However, the cron worker would take almost a year to clean up the 732,786 orphaned_security_policy_bots that are currently in the system.