[aws-cognito] Change domainPrefix after initial deploy

I wanted to change the hosted UI domain prefix after an initial deploy (`userPoolCognitoDomainPrefix`), but I'm getting a 400 on subsequent deploys. (I've just recently started to work with Cognito with CDK, so there might be some constraints in Cognito I'm not aware of. Keep up the good work!)

```
User pool already has a domain configured. (Service: AWSCognitoIdentityProviderService; Status Code: 400; Error Code: InvalidParameterException; Request ID: 8e2a6c6e-c9c3-443c-b09c-f95abc71843e; Proxy: null)
```

### Reproduction Steps

```
import * as cdk from '@aws-cdk/core';
import * as cognito from '@aws-cdk/aws-cognito';

export class IdentityStack extends cdk.Stack {
userPool: cognito.UserPool;
userPoolClientName = 'Identity';
userPoolCognitoDomainPrefix = 'identity-dev'

constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);

this.userPool = new cognito.UserPool(this, 'UserPool', {});

this.userPool.addClient('UserPoolClient', {
userPoolClientName: 'Identity',
oAuth: {
callbackUrls: [
'http://localhost:3005'
]
}
});

this.userPool.addDomain('UserPoolDomain', {
cognitoDomain: {
domainPrefix: this.userPoolCognitoDomainPrefix
}
});
}
}
```

### What did you expect to happen?

The domain should update and remove the old one as it's no longer specified.

### What actually happened?

```
identity: 1/3 | 7:55:51 AM | UPDATE_FAILED | AWS::Cognito::UserPoolDomain | UserPool/UserPoolDomain (UserPoolUserPoolDomain9F01E991)
User pool already has a domain configured. (Service: AWSCognitoIdentityProviderService; Status Code: 400; Error Code: InvalidParameterException; Request ID: 8e2a6c6e-c9c3-443c-b09c-f95abc71843e; Proxy: null)
```

### Environment

- **CLI Version :** 1.61.1
- **Framework Version:**
- **Node.js Version:** v12.18.3
- **OS :** MacOS 10.15.3
- **Language (Version):** TypeScript 3.7.5

### Other

```
identity: 1.61.1 (build 347918f)
identity: IdentityStack: deploying...
identity: IdentityStack: creating CloudFormation changeset...
identity: 0/3 | 7:55:44 AM | UPDATE_IN_PROGRESS | AWS::CloudFormation::Stack | IdentityStack User Initiated
identity: 0/3 | 7:55:49 AM | UPDATE_IN_PROGRESS | AWS::Cognito::UserPoolClient | UserPool/UserPoolClient (UserPoolUserPoolClient40176907)
identity: 1/3 | 7:55:50 AM | UPDATE_COMPLETE | AWS::Cognito::UserPoolClient | UserPool/UserPoolClient (UserPoolUserPoolClient40176907)
identity: 1/3 | 7:55:50 AM | UPDATE_IN_PROGRESS | AWS::Cognito::UserPoolDomain | UserPool/UserPoolDomain
(UserPoolUserPoolDomain9F01E991)
Requested update requires the creation of a new physical resource; hence creating one.
identity: 1/3 | 7:55:51 AM | UPDATE_FAILED | AWS::Cognito::UserPoolDomain | UserPool/UserPoolDomain (UserPoolUserPoolDomain9F01E991)
User pool already has a domain configured. (Service: AWSCognitoIdentityProviderService; Status Code: 400; Error Code: InvalidParameterException; Request ID: 8e2a6c6e-c9c3-443c-b09c-f95abc71843e; Proxy: null)
```

---

This is :bug: Bug Report

Start with the UserPool.addDomain entry point and the AWS::Cognito::UserPoolDomain behavior shown in the reproduction. Investigate how changing domainPrefix is translated during an update, then verify that changing the prefix no longer fails with the 400 error and that the previous domain is handled as expected.

Written by the indexing model from the issue text.