aws-ec2: add reference to IPv6 CIDR Blocks.

### Describe the feature

Referencing #28480 the Amazon provided IPv6 CIDR block (/56) should be able to be referenced to use it on egress records or Security Groups. Currently this is only possible for the IPv4 CIDR Block.

### Use Case

Reference the IPv6 Block directly for Security Groups and Egress Records.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

v2.137.0

### Environment details (OS name and version, etc.)

Linux

No file or test is named in the issue. Start by locating the aws-cdk EC2 handling for IPv4 CIDR blocks, then trace how those references are used by Security Groups and egress records. Done means the Amazon-provided IPv6 /56 block can be referenced in both places without regressing IPv4 behavior.

Written by the indexing model from the issue text.