fix(ec2): deploying an isolated subnet fails when `ipv6AssignAddressO…

…nCreation` is set to true (#28902)

This PR resolves the issue where deploying an isolated subnet with `ipv6AssignAddressOnCreation` enabled fails. 

### example
```ts
new Vpc(stack, 'TheVPC', {
        ipProtocol: IpProtocol.DUAL_STACK,
        subnetConfiguration: [
          {
            subnetType: testData.subnetType,
            name: 'subnetName',
            ipv6AssignAddressOnCreation: true,
          },
        ],
      });
```

### error
```sh
6:39:48 PM | CREATE_FAILED        | AWS::EC2::Subnet                      | vpcisolatedSubnet1Subnet06BBE51F
Template error: Fn::Select  cannot select nonexistent value at index 0
```

### solution
A dependency on the CidrBlock has been added [as discussed in issue](#28843 (comment)).

```ts
      (this.isolatedSubnets as PrivateSubnet[]).forEach((isolatedSubnet) => {
        if (this.ipv6CidrBlock !== undefined) {
          isolatedSubnet.node.addDependency(this.ipv6CidrBlock);
        }
      });
```


## Question
This modification results in the failure of existing integration tests.
I don't consider this change to be a breaking one, so I went ahead and updated the snapshot. Is that okay?
```sh
  CHANGED    aws-ec2/test/integ.vpc-dual-stack-ec2 0.776s
      Resources
[~] AWS::EC2::Subnet Ip6VpcDualStackPrivateSubnet1Subnet842B7F4C 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
[~] AWS::EC2::RouteTable Ip6VpcDualStackPrivateSubnet1RouteTable5326D239 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
[~] AWS::EC2::SubnetRouteTableAssociation Ip6VpcDualStackPrivateSubnet1RouteTableAssociationF1C10B6A 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
[~] AWS::EC2::Subnet Ip6VpcDualStackPrivateSubnet2SubnetEB493489 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
[~] AWS::EC2::RouteTable Ip6VpcDualStackPrivateSubnet2RouteTable56BF517C 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
[~] AWS::EC2::SubnetRouteTableAssociation Ip6VpcDualStackPrivateSubnet2RouteTableAssociationD37A3D3D 
 └─ [+] DependsOn
     └─ ["Ip6VpcDualStackipv6cidr40BE830A"]
```

Closes #28843

----

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