Clean up detection alert migrations

DEPRECATED. Cleanup API for old migration artifacts. Do not add new call sites. WARNING: This schedules deletions; ensure no production reads still point at the source index.

Migrations favor data integrity over shard size. Consequently, unused or orphaned indices are artifacts of the migration process. A successful migration can leave both the old and new indices present, so the old index may be deleted. While you can delete these indices manually, the endpoint applies a deletion policy to the relevant index, causing it to be deleted after 30 days, and removes other migration-specific artifacts.