[ECR] [request]: Allow re-encryption of a KMS encrypted repository with a different CMK

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**Tell us about your request**
Allow me to change the CMK on an existing KMS-encrypted ECR repository.

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I can change the KMS CMK from AWS managed to customer, or from one customer managed CMK to another.

**Are you currently working around this issue?**
I have to create a new repository with the KMS CMK I want to use, and then either pull images from the source repo to the new one, or push all images I want re-encrypted to the new repo.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

The issue names no repository files, tests, or code entry points. Start by reviewing the ECR repository encryption and KMS requirements described in the request, then determine whether the requested CMK change is supported by the service. Done would mean an existing KMS-encrypted repository can be re-encrypted with a different CMK without creating a replacement repository.

Written by the indexing model from the issue text.