Enable 'Prevent User Existence Error" on User Pools by Default

### Is this feature request related to a new or existing Amplify category?

auth

### Is this related to another service?

No

### Describe the feature you'd like to request

When a new User Pool is created with the Amplify CLI, it should set the "[Prevent User Existence Errors](https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-managing-errors.html)" flag on the User Pool App Clients to "true" by default.

### Describe the solution you'd like

Enable the setting by default.

### Describe alternatives you've considered

N/A

### Additional context

Requested by customer in Amplify Flutter repository issue: https://github.com/aws-amplify/amplify-flutter/issues/4170#issuecomment-1831533628

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request

### Would this feature include a breaking change?

- [X] ⚠️ This feature might incur a breaking change

The payload names no implementation files or tests; start by tracing the Amplify CLI auth User Pool creation path and its App Client configuration, then read the linked Amazon Cognito documentation. Done means newly created User Pool App Clients enable Prevent User Existence Errors by default, with coverage for the resulting configuration.

Written by the indexing model from the issue text.