[EKS] [request]: Provide platform version upgrade feature

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**Tell us about your request**
Provide platform version upgrade feature

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We just want to upgrade the EKS platform version for security patch instead of major version upgrade.
From [doc](https://docs.aws.amazon.com/eks/latest/userguide/platform-versions.html) it says
```Automatic upgrades of existing Amazon EKS platform versions are rolled out incrementally. The roll-out process might take some time. If you need the latest Amazon EKS platform version features immediately, you should create a new Amazon EKS cluster.```

I don’t know when the automatic update will be actually performed. For example, the latest platform ver for 1.15 is already ```eks.4``` , however all our 1.15 clusters still stay at ```eks.2``` , there is no automatic upgrade performed.

Even if automatic upgrade is performed, i still prefer manual upgrade, It would be better to let user decide when to upgrade the control plane. Automatic upgrade is kind of risky.

The request concerns Amazon EKS platform versions and links to the AWS platform-version documentation; no repository files, tests, or implementation entry points are mentioned. Start by reviewing that documentation and the issue discussion, then define what a manual control-plane platform-version upgrade would expose and how completion would be verified.

Written by the indexing model from the issue text.