(ecs-patterns): omiting cluster and specifying only vpc results in an error

### What is the problem?

The ecs-patterns README states that 'Instead of providing a cluster you can specify a VPC and CDK will create a new ECS cluster.' However, omiting the cluster prop and only provided a vpc results in an error. And there is not clear direction on how to do this properly.

So far I have noticed this issue with the following constructs. It might apply to more.
- NetworkMultipleTargetGroupsEc2Service
- ApplicationMultipleTargetGroupsEc2Service
- ApplicationLoadBalancedEc2Service

This might just be a miss in the documentation, and not missing functionality. I will need to dive deeper into this to find out.

### Reproduction Steps

Synthesize and app with this sample code:

```
const stack = new Stack();
const vpc = new Vpc(stack, 'VPC');

const service = new ApplicationMultipleTargetGroupsEc2Service(stack, 'Service', {
vpc,
memoryLimitMiB: 1024,
taskImageOptions: {
image: ContainerImage.fromRegistry('test'),
},
});
```

### What did you expect to happen?

No errors, and it synthesizes an ECS service with a cluster created by the CDK.

### What actually happened?

I get this error:
```
[Default/Service/Service] Cluster for this service needs Ec2 capacity. Call addXxxCapacity() on the cluster.
```

### CDK CLI Version

1.139.0

### Framework Version

_No response_

### Node.js Version

12.22.7

### OS

MacOS

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start with the ecs-patterns README and the ApplicationMultipleTargetGroupsEc2Service, NetworkMultipleTargetGroupsEc2Service, and ApplicationLoadBalancedEc2Service constructs. Synthesize the provided VPC-only TypeScript example and trace the “Cluster for this service needs Ec2 capacity” error. Done means either the documented VPC-only usage synthesizes successfully or the README clearly explains the required configuration.

Written by the indexing model from the issue text.