Cleanup Issue WorkItemType ID backfill MR

!71869 (merged) introduced 2 temporary solutions to make the data migration perform better. After that change is released we should perform a a couple of cleanup actions on the next release:

• Run ensure_batched_background_migration_is_finished for each of the background migrations
• Remove temporary index tmp_index_issues_on_issue_type_and_id
• Add foreign key index back to issues table on the work_item_type_id column