Remove the rule for region cn-north-1 which limits NumberOfAZs by 2

[kevinylq]

Hello,

According to the post, cn-north-1 now has 3 AZs.
https://aws.amazon.com/about-aws/whats-new/2021/06/aws-china-beijing-region-adds-the-third-availability-zone/?nc1=h_ls

Could you please exclude below rule from the template?

Rules:
AZRule:
RuleCondition: !Equals [ !Ref 'AWS::Region', cn-north-1 ]
Assertions:
- Assert: !Equals [ !Ref NumberOfAZs, '3' ]
AssertDescription: To enable NAT gateways you must have both CreatePrivateSubnets and CreatePublicSubnets set to 'true'