Iterate over `epics` table and create corresponding records in the `work_item_parent_links` table based on `epics.parent_id` relationship. In order to sync `epics` records into `work_item_parent_links` we need to find the corresponding `epics.issue_id` for every `epics.parent_id` which would act as the `work_item_parent_links.work_item_parent_id` and `epics.issue_id` would be mapped to `work_item_parent_links.work_item_id` This migration will have 2 parts: * Enqueue the backfilling migration * Finalise the backfilling migration This migration is blocked by https://gitlab.com/groups/gitlab-org/-/epics/12740+ migration as we need to make sure that every epic has a corresponding record in issues table.