chore(cognito): docs showed incorrect default value for preventUserEx…

…istenceErrors (#17667)

The docs for preventUserExistenceErrors showed a default of `true` when
the default has always been `false`. Update the docs to reflect the
actual default. Also remove references to "new stacks" since the
functionality is the same between new and existing stacks.

fixes #17044


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packages/@aws-cdk/aws-cognito/README.md

@@ -582,7 +582,7 @@ pool.addClient('app-client', {
 An app client can be configured to prevent user existence errors. This
 instructs the Cognito authentication API to return generic authentication
 failure responses instead of an UserNotFoundException. By default, the flag
-is not set, which means different things for existing and new stacks. See the
+is not set, which means the CloudFormation default (false) will be used. See the
 [documentation](https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-managing-errors.html)
 for the full details on the behavior of this flag.
 

packages/@aws-cdk/aws-cognito/lib/user-pool-client.ts

@@ -243,7 +243,7 @@ export interface UserPoolClientOptions {
    * user does not exist in the user pool (false), or whether it returns
    * another type of error that doesn't reveal the user's absence.
    * @see https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-managing-errors.html
-   * @default true for new stacks
+   * @default false
    */
   readonly preventUserExistenceErrors?: boolean;