Single stack IPv6 Support

[juanluisvaladas]

Currently k0s doesn't support IPv6 single stack, IPv6 only works on dualStack installations.
This was requested in the past and there are few issues and pull requests related to this such as #1726

Known issues to be solved:

• Have a proper single stack test (might need to be airgapped due to github limitations)
• CoreDNS is known to have some issues with its configuration
• Sane defaults (including but not limited to Default IPv6podCIDR value in the documentation doesn't make much sense #5844)
• Configuration validation (including but not limited to Make missing node IPs in dual-stack mode a hard error #5840)
• Documentation
• kube-controllers-manager node CIDRs
• Introduce k0s Feature Gates #6084

Known unknowns:

• Calico
• Kube router
• Kube proxy
• CPLB
  • IPVS
  • Userspace
• NLLB
• Network conformance tests
  • Calico
  • Kuberouter

We'll also need to run the conformance tests but most likely we'll need to delay it to 1.35.

[juanluisvaladas]

If someone wants to track this, I'm working on this issue in my local branch https://github.com/juanluisvaladas/k0s/tree/ipv6-singlestack.

[juanluisvaladas]

I managed to run locally the network conformance tests on calico IPv6 single stack. Didn't manage so with kube-router.

I'm just going to leave here some quick notes:
1- The image list is acquired programatically. The source of truth is here: https://github.com/kubernetes/kubernetes/blob/v1.33.3/test/utils/image/manifest.go#L70
2- I need to double check but I believe sonobuoy doesn't have anything like an image list command
3-If we can't use anything off the shelf from sonobuoy, the ideal thing to do is to implement it on sonobuoy, the fast thing to do is to do something like we do with hack/gen-bindata/gen_bindata.go