[EKS] [request]: Managed Ubuntu Node Groups

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**Tell us about your request**
Add Ubuntu AMI support for EKS Managed Node Groups

**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 have workloads that require Ubuntu worker nodes, and we'd like to run them as Managed Nodes.

**Are you currently working around this issue?**
Managing Ubuntu node groups ourselves through eksctl.

**Additional context**
The Ubuntu EKS AMI is provided together with partner Canonical.

Start by reading the EKS Managed Node Groups context and the described eksctl workaround. Confirm how the Ubuntu EKS AMI supplied by Canonical fits the requested support; done means Ubuntu worker nodes can run as EKS Managed Nodes.

Written by the indexing model from the issue text.