[bug] Deprecate use of eks v1.12 references in cncfekscluster.sh

**Summary**

The script `cncfekscluster.sh` https://github.com/cncf/devstats-helm-example/blob/d9481665693eced90801e3b2e0e12083cd2c3970/scripts/cncfekscluster.sh uses a deprecated version of Amazon's EKS service, `v1.12`

> As of today, Kubernetes version 1.12 is deprecated in EKS, and will no longer be supported on May 11th, 2020. On this day, you will no longer be able to create new 1.12 clusters and all EKS clusters running Kubernetes version 1.12 will be updated to the latest available platform version of Kubernetes version 1.13.

[Amazon EKS now supports Kubernetes version 1.15](https://aws.amazon.com/about-aws/whats-new/2020/03/amazon-eks-now-supports-kubernetes-version-1-15/)

**Action Item**

- [ ] upgrade references to latest AWS EKS version (`v1.15`?)

Apri scripts/cncfekscluster.sh e individua i riferimenti ad Amazon EKS che utilizzano Kubernetes v1.12. Aggiorna questi riferimenti all'ultima versione di AWS EKS identificata dall'issue, quindi verifica che nello script non rimanga alcun riferimento obsoleto a v1.12.

Scritto dal modello di indicizzazione a partire dal testo della issue.