Ensure that IPv6 cluster cannot be created if `vpc.NAT` configuration is set

[nikimanoledaki]

Part of #4255

Description
Configuring VPC.NAT is not supported on ipv6 clusters, we should present a nice error to the user when this happens.

Scope

• Add some validation that both fields are not set at the same time

Acceptance Criteria

• As an eksctl user
• When I set ipFamily to ipv6 and vpc.nat to
• I get a nice error message telling me that vpc.nat is not supported with ipv6