aws-rds: add Aurora MySQL engine version 3.13.0

### Describe the feature

Expose Aurora MySQL 3.13.0, compatible with MySQL 8.0.45, as a built-in `AuroraMysqlEngineVersion` constant. The engine version became generally available on August 28, 2026.

### Use Case

CDK users should be able to select Aurora MySQL 3.13.0 through the same discoverable, strongly typed API used for earlier Aurora MySQL 3.x releases, with the correct `aurora-mysql8.0` parameter group family.

### Proposed Solution

Add `AuroraMysqlEngineVersion.VER_3_13_0` using `AuroraMysqlEngineVersion.builtIn_8_0('3.13.0')` and include it in the parameter group family test matrix.

Implementation is available in #38736.

### Other Information

- AWS announcement: [Amazon Aurora MySQL 3.13 (compatible with MySQL 8.0.45) is generally available](https://aws.amazon.com/about-aws/whats-new/2026/08/amazon-aurora-mysql-313-available/)
- The current latest built-in Aurora MySQL 3.x constant is `VER_3_12_0`.

### Acknowledgements

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

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

Current `main` branch as of September 1, 2026.

### AWS CDK CLI version

N/A — API constant request.

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

N/A — API constant request.

Start at the AuroraMysqlEngineVersion API and the parameter group family test matrix named in the issue. Confirm the new built-in constant uses the 3.13.0 engine version and that the matrix covers the aurora-mysql8.0 family; implementation is already available in pull request #38736.

Written by the indexing model from the issue text.