Add more accurate way of counting background migrations
What does this MR do?
Sidekiq::Queue only counts currently enqueued jobs. For background migrations, most jobs are instead scheduled and therefore in Sidekiq::ScheduledSet. This is because if we enqueued them all, we'd process them as fast as Sidekiq could pick them up, which could:
- Block other jobs from processing.
- Overwhelm the database.
Closes #204769 (closed).