service: migration_task: Implement using migration_manager::merge_schema_from()
service: migration_task: Implement using migration_manager::merge_schema_from()

To avoid duplication.

duarten/scylla,kjniemi/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,raphaelsc/scylla