Adds a new cop to check the use of finalize_background_migration method

It prevents the usage of this deprecated method

.rubocop.yml

@@ -935,6 +935,10 @@ Migration/BackgroundMigrationBaseClass:
 BackgroundMigration/AvoidSilentRescueExceptions:
   Enabled: false
 
+Migration/AvoidFinalizeBackgroundMigration:
+  Include:
+    - 'db/post_migrate/*.rb'
+
 Style/ClassAndModuleChildren:
   Enabled: true
 

.rubocop_todo/migration/avoid_finalize_background_migration.yml

+---
+Migration/AvoidFinalizeBackgroundMigration:
+  Details: grace period
+  Exclude:
+    - 'db/post_migrate/20220502015011_clean_up_fix_merge_request_diff_commit_users.rb'
+    - 'db/post_migrate/20220525131557_cleanup_backfill_integrations_enable_ssl_verification.rb'
+    - 'db/post_migrate/20220713133515_cleanup_backfill_draft_statuses_on_merge_requests.rb'

rubocop/cop/migration/avoid_finalize_background_migration.rb

+# frozen_string_literal: true
+
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      class AvoidFinalizeBackgroundMigration < RuboCop::Cop::Base
+        include MigrationHelpers
+
+        RESTRICT_ON_SEND = [:finalize_background_migration].freeze
+
+        MSG = 'Prefer `ensure_batched_background_migration_is_finished` over ' \
+              '`finalize_background_migration` in Batched Background Migrations. ' \
+              'See https://docs.gitlab.com/ee/development/database/batched_background_migrations.html'
+
+        def on_send(node)
+          add_offense(node)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/migration/avoid_finalize_background_migration_spec.rb

+# frozen_string_literal: true
+
+require 'rubocop_spec_helper'
+require_relative '../../../../rubocop/cop/migration/avoid_finalize_background_migration'
+
+RSpec.describe RuboCop::Cop::Migration::AvoidFinalizeBackgroundMigration, feature_category: :database do
+  context 'when file is under db/post_migration' do
+    it "flags the use of 'finalize_background_migration' method" do
+      expect_offense(<<~RUBY)
+        class FinalizeMyMigration < Gitlab::Database::Migration[2.1]
+           MIGRATION = 'MyMigration'
+
+           def up
+             finalize_background_migration(MIGRATION)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{described_class::MSG}
+           end
+        end
+      RUBY
+    end
+  end
+end