Add TaskId method to GetTaskDescriptor

This commit adds the TaskId method to GetTaskDescriptor to
maintain binary compatibility. The REST API specs have been updated
to make task_id a required parameter, which means that the
constructor accepts the task_id.

Author: russcam

src/Nest/Cluster/TaskManagement/GetTask/GetTaskRequest.cs

@@ -15,5 +15,8 @@ public partial class GetTaskDescriptor
 	{
 		[Obsolete("Maintained for binary compatibility. Use the constructor that accepts a task id. Will be removed in 7.0")]
 		public GetTaskDescriptor() { }
+
+		[Obsolete("Maintained for binary compatibility. Use the constructor that accepts a task id. Will be removed in 7.0")]
+		public GetTaskDescriptor TaskId(TaskId taskId) => this.Assign(a => a.RouteValues.Required("task_id", taskId));
 	}
 }