UserPoolClient - Retrieving the client secret requires an unnecessary custom resource

### Describe the bug

A change was merged previously to CDK to support a workaround for getting the client secret. This custom resource requires someone deploying a stack with only cognito to have a VPC attachment to the custom resource for accounts with SCPs limiting lambdas to have to run in a VPC.

### Expected Behavior

Native CFN is used to get Cognito Client Secrets

### Current Behavior

As implemented here, a custom resource is used to get the client secret:
https://github.com/aws/aws-cdk/pull/21262/files#diff-9713362aa6af827d0bf2a8c68319b5bb9c74f888f9ab417266ff1b98aa121ae2R429

### Reproduction Steps

create a cognito app client w/ secret

```python
self.client = user_pool.add_client(
"service-client",
user_pool_client_name="service-client",
supported_identity_providers=[
aws_cognito.UserPoolClientIdentityProvider.COGNITO
],
o_auth=self.o_auth_settings,
auth_flows=aws_cognito.AuthFlow(
user_srp=True,
),
refresh_token_validity=Duration.days(1),
generate_secret=True,
)

self.my_secret = SecretValue.unsafe_unwrap(self.client.user_pool_client_secret)
```

### Possible Solution

Since the CFN Attribute was fixed in 2023,
Remove the custom resource and instead generate the following CFN during synth to access the client secret:

```json
"UserPoolClientIdSecret":{
"Value": {
"Fn::GetAtt": ["CognitoUserPoolClient", "ClientSecret"]
}
}
```

### Additional Information/Context

_No response_

### CDK CLI Version

2.117.0

### Framework Version

_No response_

### Node.js Version

v18.17.1

### OS

OSX Sonoma

### Language

Python

### Language Version

3.10.11

### Other information

_No response_

Start at the Cognito UserPoolClient implementation and the custom-resource path identified by the linked pull request diff. Verify how the client secret is exposed during synthesis, then confirm the resulting CloudFormation uses the Cognito ClientSecret Fn::GetAtt and no longer requires the custom resource or VPC attachment.

Written by the indexing model from the issue text.