[eks] [request]: Use correct SemVer for Kubernetes version

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**Tell us about your request**
I'd like the EKS Kubernetes version to be a valid Semver 2 version, meaning no pre-release segment if it's not a pre-release.

**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?**
The current Kubernetes version for EKS is an incorrect pre-release SemVer version (e.g. `v1.18.9-eks-d1db3c`) and causes any automation expecting a valid SemVer to fail. It looks like this is because you're trying to store two pieces of data in a variable only capable of storing one.

This means that Helm charts which use `.Capabilities.KubeVersion.Version` for conditional logic either need to provide a static override or manually sanitise the version to remove the pre-release segment.

Releated issue https://github.com/prometheus-community/helm-charts/issues/1071.

**Are you currently working around this issue?**
For Helm this is only possible in some scenarios when the author of the chart has already come across this issue and has enabled the version to be set manually or sanitises the version to ignore pre-release segments.

**Additional context**
n/a

**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.)

Start by locating where the EKS Kubernetes version is exposed and how Helm receives `.Capabilities.KubeVersion.Version`. Reproduce the invalid version format, then verify the reported value follows SemVer 2 without a pre-release segment when the Kubernetes release is not pre-release.

Written by the indexing model from the issue text.