EKS - fix secrets encryption with KMS key checkbox

[mantis-toboggan-md]

Summary

Fixes #11558

Occurred changes and/or fixed issues

This PR updates the EKS provisioning form to set eksConfig.secretsEncryption when the 'Encrypt Secrets with a KMS key' checkbox is checked.

Areas or cases that should be tested

1. Create an EKS cluster, check the 'Encrypt secrets' checkbox and select a key. Verify that the save request includes eksConfig.secrets.Encryption: true
2. Wait for the cluster to provision; edit the cluster and verify that the kms key arn is shown in the form ,but is not editable

Screenshot/Video

Checklist

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[jordojordo]

lgtm