Finalize background migration to backfill packages_composer_packages table

Description

In Backfill packages_composer_packages table (#548756 - closed) we added the background migration to backfill packages_composer_packages table. After the migration was included in the required stop for self-managed users we should finalize it https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration