Modifies which task set in a service is the primary task set. Any parameters that are updated on the primary task set in a service will transition to the service. This is used when a service uses the <code>EXTERNAL</code> deployment controller type. For more information, see <a href="https://docs.aws.amazon.com/AmazonECS/latest/developerguide/deployment-types.html">Amazon ECS Deployment Types</a> in the <i>Amazon Elastic Container Service Developer Guide</i>.