feat(batch): ec2 Managed Compute Environment support default instance classes, deprecate useOptimalInstanceClasses (#35537)

### Issue # (if applicable) Closes #35515 ### Reason for this change - https://aws.amazon.com/blogs/hpc/introducing-default-instance-categories-for-aws-batch/ - `optimal` behaviour change in early November 2025 ### Description of changes - EC2 Managed Compute Environment support default instance classes - Deprecate `useOptimalInstanceClasses` ### Description of how you validated changes Unit + Integ ### Checklist - [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md) ---- *By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: phuhung273