Update partitioning helpers to use batched background migrations

What?

We want to update all the existing partitioning helpers to use batched background migrations instead of regular (Sidekiq) background migrations when we copy the data from the original table to the partitioned table.

Deliverables:

• Complete #379126 (closed)
• Complete !116601 (merged)
• Update docs to not refer to legacy background migrations.
  • https://docs.gitlab.com/ee/development/database/migrations_for_multiple_databases.html
  • https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html