Ensure that IPv6 can only be set for k8s >= 1.21

[Callisto13]

Part of #4255

Description
If ipFamily is set to ipv6, ensure that the version is >= 1.21.

metadata:
  name: <cluster-name>
  version: "1.21"

vpc:
   ipFamily: ipv6

Scope

• Add some validation for when ipFamily is set
• Error out if the version is not supported

Acceptance Criteria
Story 1

• As an eksctl user
• When I set ipFamily to ipv6 and metadata.version to "1.21"
• I can see in the console that I have created an IPv6 VPC+cluster
• I can learn about this in the docs

Story 2

• As an eksctl user
• When I set ipFamily to ipv6 and metadata.version to "1.20"
• I see an error that this feature is not supported with this version