Backfill `detected_at` values from `created_at` column on Vulnerabilites

Why are we doing this work?

After !66164 (merged) gets merged we will need to backfill values for detected_at based on created_at column.

Implementation plan

1. database Create a background migration that copies created_at column to detected_at column
2. database Create a post-deployment migration that schedules the background migration created in step 1 only for records where detected_at IS NULL