Remove old Background Migration framework

Problem

We need to deprecate and remove the old Background framework as it is no longer being used and it's causing some confusion in the codebase.

Solution

• Remove legacy background migration helpers
• Remove existing cops no longer being used (background_migrations)
• Remove model and associated table
• Remove/Update docs related to them,
  • https://docs.gitlab.com/ee/development/database/background_migrations.html
  • https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#legacy-background-migrations
  • https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html#be_finalize_background_migration_of

cc @praba.m7n