(Cognito User Pool): signInCaseSensitive default value contradicts service recommendation

### Describe the feature

Amazon Cognito user pools created from console, APIs or CLI are case insensitive by default (signInCaseSensitive is false by default), this is more secure and the recommended default by the service.

CDK sets this flag to true by default which contradicts service default recommendation.

### Use Case

Creating new user pool should create a case insensitive user pool by default.

### Proposed Solution

Change signInCaseSensitive default value to false.

### Other Information

https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-case-sensitivity.html

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.58.1

### Environment details (OS name and version, etc.)

all environments

No file or test is identified in the issue. Locate the Cognito user-pool implementation and its signInCaseSensitive default, then inspect related tests before changing the default; done means newly created user pools use false by default and the test expectations reflect that behavior.

Written by the indexing model from the issue text.