Reschedule artifact expiry backfill
What does this MR do?
Based on the plan described in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10177#note_516129368. We first updated the index used in the background migration with !55091 (merged). This MR reschedules the background migration to run again and backfill the remaining 11M that weren't processed the first time, using the new index.
There's an extensive description of the reasoning behind the scheduling logic (the reasoning why we're using this specific query, the inner and outer batch sizes, etc.) in !47723 (merged)
We originally estimated 80h for the initial background migration that was supposed to process ~160M records. This is supposed to process 11M records so my guess is that this is going to take 5.5h or less.
Related https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10177