AWS::EC2::Subnet - False-positive drift on Ipv6CidrBlock property

### Name of the resource

AWS::EC2::Subnet

### Issue Description

Drift detection falsely says that the Ipv6CidrBlock property's value is drifted.

### Expected Behavior

The values returned in the drift report should both reflect the same version of the Ipv6CidrBlock property's value.

### Observed Behavior

The false-positive drift appears to be because a canonical version of the CIDR is returned and compared to a non-canonical version of it, in the drift evaluation workflow.

### Test Cases

Deploy the CloudFormation template below and immediately run drift detection on the stack:
```
Resources:
VPC:
Type: AWS::EC2::VPC
Properties:
CidrBlock: 10.0.0.0/16
EnableDnsHostnames: true
EnableDnsSupport: true
InstanceTenancy: default

IPV6CidrBlock:
Type: AWS::EC2::VPCCidrBlock
Properties:
VpcId: !Ref VPC
AmazonProvidedIpv6CidrBlock: true

Subnet:
Type: AWS::EC2::Subnet
Properties:
VpcId: !Ref VPC
AvailabilityZone: !Select
- 0
- !GetAZs
Ref: AWS::Region
Ipv6Native: true
Ipv6CidrBlock: !Select
- 0
- !Cidr
- !Select
- 0
- !GetAtt VPC.Ipv6CidrBlocks
- 1
- 64
AssignIpv6AddressOnCreation: true
MapPublicIpOnLaunch: false
```

Start with the CloudFormation template in the issue: deploy it and immediately run stack drift detection, then inspect the drift report for AWS::EC2::Subnet.Ipv6CidrBlock. Done means the reported expected and actual values use the same canonical form and no false-positive drift is reported.

Written by the indexing model from the issue text.