Ensure that IPv6 cluster cannot be created if `autoAllocateIPv6` is set

[nikimanoledaki]

Part of #4255
Description
We have an old legacy IPV6 field vpc.autoAllocateIPV6. This field is redundant with IPV6 clusters. Lets error when the user sets this to true when creating an IPV6 cluster

Scope

• Validation that vpc.autoAllocateIPV6 isn't set when creating a IPV6 cluster

Acceptance Criteria

• As an eksctl user
• When I set ipFamily to ipv6 and vpc.autoAllocateIPV6 to true
• I get a nice error message telling me that vpc.autoAllocateIPV6 is not supported with ipv6
• I should learn about this in the docs