fix(cognito): standard attr timezone unexpectedly creates custom attr

[nija-at]

List of standard attributes -
https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-settings-attributes.html#cognito-user-pools-standard-attributes

fixes #5866

---

Note to reviewer: Unit test has been intentionally left out since the way this attribute is used is incomplete. Currently, the way users are using this attribute is in combination with Cfn* classes -

new cognito.CfnUserPool(this, "UserPool", {
  schema: [
    {
      name: cognito.UserPoolAttribute.EMAIL,
      attributeDataType: "String",
      required: true,
      mutable: true
    },
    {
      name: cognito.UserPoolAttribute.TIMEZONE,
      attributeDataType: "String",
      required: false,
      mutable: true
    }
  ]
});

All of this should get fixed up with the implementation of - aws/aws-cdk-rfcs#91

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

AWS CodeBuild CI Report

• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[mergify]

Thank you for contributing! Your pull request is now being automatically merged.