Document IPv6 only clusters incomaptible with kube-vip < 0.7.2

[65278]

Describe the solution you'd like
kube-vip tries to define the cluster interface by looking for the default route. For versions < v0.7.2, this does not work in IPv6 only environments because IPv6 routing tables are not considered:

root@ipv6test01-control-plane-5000b-mg75w:~# crictl logs d1dd2691d6aab
time="2024-03-14T11:48:58Z" level=info msg="Starting kube-vip.io [v0.5.10]"
time="2024-03-14T11:48:58Z" level=info msg="namespace [kube-system], Mode: [ARP], Features(s): Control Plane:[true], Services:[false]"
time="2024-03-14T11:48:58Z" level=info msg="No interface is specified for VIP in config, auto-detecting default Interface"
....
time="2024-03-14T11:52:30Z" level=fatal msg="unable to detect default interface -> [Unable to find default route]"

Document this (and the kube vip default interface template variable) for future releases.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-proxmox version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[wikkyk]

done #172