Update the Kubernetes version for creating an Integrated EKS Cluster

What does this MR do?

Remove the 1.15 k8s version cause it is not supported anymore.
Update the cloudformation stack with 1.20 version.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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