Rename RuboCop rule Migration/BackgroundMigrationBaseClass to Migration/BatchedMigrationBaseClass

The following discussion from !154569 (merged) should be addressed:

• @splattael started a discussion:

  Question (non-blocking) I wonder why this cop rule is called _Background_Migration instead of _Batched_Migration? 🤔

        class BatchedMigrationBaseClass < RuboCop::Cop::Base

  Not this MR though!

Implementation Guide

Rename usages of BackgroundMigrationBaseClass across the codebase to BatchedMigrationBaseClass this includes:

• rubocop/cop/migration/background_migration_base_class.rb
• lib/gitlab/background_migration/
• .rubocop.yml
• spec/rubocop/cop/migration/background_migration_base_class_spec.rb
• ee/lib/ee/gitlab/background_migration/.rubocop.yml
• .rubocop_todo/migration/background_migration_base_class.yml