Ensure that IPv6 cluster cannot be created if `serviceIPv4CIDR` is set

[nikimanoledaki]

Part of #4255

Description
Fail if ipFamily is set to ipv6 and kubernetesNetworkConfig.serviceIPv4CIDR is set at all. This will not be configurable as an input when creating an IPv6 cluster.

Acceptance Criteria
As an eksctl user
When I try to create a cluster and set both vpc.ipFamily and kubernetesNetworkConfig.serviceIPv4CIDR in my configuration file
I should see a nice error message
I should also be able to learn about this in the docs