Ensure that unmanaged nodegroups cannot be created for IPv6 cluster

[nikimanoledaki]

Part of #4255

Description
Unmanaged nodegroups will not be supported initially for IPv6 clusters. We are slowly dropping support for unmanaged nodegroups in favour of managed nodegroups so it makes sense to drop support for them when it comes with the IPv6 feature. This may change in the future if there is demand for it.

Scope

• Fail if cluster is being created with a config file that contains unmanaged nodegroup(s)
• Fail if unmanaged nodegroup is being created for IPv6 cluster

Acceptance Criteria
Story A

• As an eksctl user
• When I try to create a cluster and my config file contains unmanaged nodegroups
• I should see a nice error message
• I should be able to read about this in the docs

Story B

• As an eksctl user
• When I try to create an unmanaged nodegroup for my IPv6 cluster
• I should see a nice error message
• I should be able to read about this in the docs

[Skarlso]

I can't add dependencies... someone please add eksctl#4259 as a dep to this ticket.