(elasticloadbalancingv2): cannot have more than one target group on a single NLB listener

NLB listeners currently allow for more than one target group to be added: https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-elasticloadbalancingv2/lib/nlb/network-listener.ts#L33

However, there can only be one target group for an NLB listener since there can only be one default rule for an NLB listener: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html

Reproduction Steps

import * as cdk from '@aws-cdk/core';
import * as ecs from '@aws-cdk/aws-ecs';
import * as elb from '@aws-cdk/aws-elasticloadbalancingv2';
import { Vpc } from '@aws-cdk/aws-ec2';
export class NlbTestStack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);
    const vpc = new Vpc(this, 'Vpc');
    const cluster = new ecs.Cluster(this, 'Cluster', {
      vpc
    });
    const taskDefinition = new ecs.TaskDefinition(this, 'TaskDef', {
      compatibility: ecs.Compatibility.FARGATE,
      cpu: "1024",
      memoryMiB: "2048",
    });
    const container = taskDefinition.addContainer('container', {
      image: ecs.ContainerImage.fromRegistry('node'),
    });
    container.addPortMappings({ containerPort: 80 });
    const service1 = new ecs.FargateService(this, 'Service1', {
      cluster,
      taskDefinition,
    });
    const service2 = new ecs.FargateService(this, 'Service2', {
      cluster,
      taskDefinition,
    });
    const lb = new elb.NetworkLoadBalancer(this, 'NLB', {
      vpc,
    })
    const listener = lb.addListener('listener1', {
      port: 80,
    });
    listener.addTargets('service1target', {
      port: 80,
      targets: [service1]
    })
    listener.addTargets('service2target', {
      port: 80,
      targets: [service2]
    })
  }
}

Deploying this stack results in the following CFN error:

Invalid request provided: CreateService error: The target group with targetGroupArn arn:aws:elasticloadbalancing:us-west-1:430030518091:targetgroup/NlbTe-NLBli-1XYX36JEGVQHP/010a21c45e3d17ea does not have an associated load balancer. (Service: Ecs, Status Code: 400, Request ID: 4f3d5f5f-9a56-4af9-8a2f-673267b35bb3, Extended Request ID: null)

What did you expect to happen?

What actually happened?

Environment

• CDK CLI Version : 1.77
• Framework Version: 1.77
• Node.js Version: v12.15.0
• OS : MacOS
• Language (Version): Typescript 3.8.3

Other

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This is 🐛 Bug Report

[babebort]

up ingress Failed deploy model due to failed to create listener rule: TargetGroupAssociationLimit: The following target groups cannot be associated with more than one load balancer: arn:aws:elasticloadbalancing:eu-central-1:12222222:targetgroup/k8s-default-monorepo-bafd567687/add252b99f892cc7 status code: 400, request id: 4aff53e2-9c52-4dc9-85d0-a8dbd84e5e21

[github-actions[bot]]

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