Validate if registry mirror is updatable during cluster upgrade

**What would you like to be added**:

Based on current EKS-A validation logic, registryMirror is not an immutable field. But we do not have any e2e tests validating that updating a registryMirror will work as expected. EKS-A official doc also does not mark the field as upgradable https://anywhere.eks.amazonaws.com/docs/tasks/cluster/cluster-upgrades/vsphere-and-cloudstack-upgrades/#upgradeable-cluster-attributes

We need to validate and see if adding/updating/deleting registry mirror field will work or not. Base on the result, either add immutability check to validation code if worked, OR add e2e tests for the feature and update eks-a doc.

**Why is this needed**:

Start by reviewing the current EKS-A validation logic and the documented upgradeable cluster attributes. Check whether adding, updating, and deleting registryMirror works during a cluster upgrade; done means applying the appropriate immutability validation, or adding e2e coverage and updating the EKS-A documentation based on the result.

Written by the indexing model from the issue text.