[EKS] [request]: Free IPs in subnet for CP upgrade

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**Tell us about your request**
Could you change the number of free IPs needed in a subnet to launch a control-plane update to 3, as stated by the [documentation](https://docs.aws.amazon.com/eks/latest/userguide/update-cluster.html) ?

**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?**
In order to manage the number of IPs consumed by our EKS architecture, we have limited our control-plane/nodes subnets to 3 /28 CIDR Blocks. We assumed from this architecture that we could manage up to at least 20 nodes without being dependent on node rolling out strategy.
Sadly, on cluster with more than 15 nodes, control-plane update fails with the following error message : `Error: error updating EKS Cluster (my-cluster) version: InvalidRequestException: Provided subnets subnet-xxxxxxxxxx Free IPs: 4 subnet-xxxxxxxx Free IPs: 4 , need at least 5 IPs in each subnet to be free for this operation`
What bums me most is that [documentation](https://docs.aws.amazon.com/eks/latest/userguide/update-cluster.html) states that "To update the cluster, Amazon EKS requires two to three free IP addresses from the subnets", which would meet our goals, and yet the limit has been set to 5.

**Are you currently working around this issue?**
We have to downscale our cluster until the control-plane update is in progress.

**Additional context**
Ultimately https://github.com/aws/containers-roadmap/issues/170 would be a nice addition for us to manage any subnet size related limitation.

**Attachments**
N/A

Start with the linked Amazon EKS update-cluster documentation and the reported error showing that each subnet needs five free IPs. Review the related containers-roadmap issue 170 for context; done would mean control-plane updates require only the documented two to three free IP addresses per subnet.

Written by the indexing model from the issue text.