(rds): add missing AuroraMysqlEngineVersion version 8.4.7 to cdk

### Describe the feature

add missing AuroraMysqlEngineVersion version 8.4.7 to rds cluster engines

### Use Case

I want to use CDK to create a Aurora Database with the latest engine version

### Proposed Solution

_No response_

### Other Information

I created [a Pull Request](https://github.com/aws/aws-cdk/pull/37923). That caused integration tests tofail, because they use the latest version of mysql aurora (that changed from 8.0 to 8.4). That's too much for my limited abilities to handle.

### Acknowledgements

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

### AWS CDK Library version (aws-cdk-lib)

2.254.0

### AWS CDK CLI version

2.1122.0 (build c8f270c)

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

MacOS Tahoe 26.5 (25F71)

Start by locating the RDS cluster-engine version definitions for Aurora MySQL and the integration tests that select the latest Aurora MySQL version. Confirm how the tests behave when latest changes from 8.0 to 8.4, then verify the new 8.4.7 option and passing integration coverage; the issue already links PR #37923, so coordinate with that work rather than starting duplicate changes.

Written by the indexing model from the issue text.