feat(ec2): add C8a (InstanceClass.C8A) to available instance classes

[syukawa-gh]

Issue

Closes #36722

Reason for this change

The C8a instance class (compute-optimized, based on 5th generation AMD EPYC processors) is available on AWS but was missing from the CDK InstanceClass enum.

Description of changes

• Added COMPUTE8_AMD and C8A enum values to InstanceClass
• Added corresponding entries in the instance class mapping table

Description of how you validated changes

Follows the same pattern as other instance class additions (e.g., C7A, C8G, C8I).

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[syukawa-gh]

Exemption Request: The EC2 README does not contain an enumeration of instance classes, so there is no natural place to document the addition of C8A. The instance class is self-documenting through the enum JSDoc and follows the same pattern as all other instance class additions (C7A, C8G, C8I, etc.).

[syukawa-gh]

Exemption Request: This change is a minimal fix that does not alter CloudFormation resource behavior in a way that requires a new integration test. The fix is covered by unit tests.

[syukawa-gh]

Updated: Added C8A instance class example to the EC2 README. The previous Exemption Request for the README change is no longer needed.