[ECS Cluster] [bug?]: ECS::Cluster update behavior removes existing settings

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
I have an existing ECS cluster deployed with cloudformation with containerInsights enabled. After also adding support for the ECS exec feature in my cloudformation template, I noticed that the settings were cleared from the cluster. I assume this is related to cloudformations implementation of the update behavior for cluster.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
See first paragraph. It's basically a bug I think.

**Are you currently working around this issue?**
Manually update the settings after updating the cluster.

Start by reproducing the reported CloudFormation update: add ECS Exec support to an existing cluster with containerInsights enabled and observe whether the setting is cleared. Done means updating the cluster preserves its existing settings while adding ECS Exec support.

Written by the indexing model from the issue text.