Remove the Cillium IPv6 checkbox and add a flannel ipv6 checkbox #15734
Description
The UI has a checkbox to enable IPv6 support when the selected cni is cillium or multus,cillium. RKE2 provisioning with cillium no longer requires this extra configuration to support IPv6. The UI should remove this checkbox and all associated logic.
To provision k3s clusters with ipv6 enabled, users generally need to set --flannel-ipv6-masq - see https://docs.k3s.io/networking/basic-network-options#single-stack-ipv6-networking
When using IPv6 addresses that are not publicly routed, for example in the ULA range, you might want to add the --flannel-ipv6-masq option to enable IPv6 NAT, as per default pods use their pod IPv6 address for outgoing traffic.
The ui should allow users to set this server arg and the UI should set it by default when the UI detects that the cluster is ipv6/dual-stack. The UI will need to add a checkbox that toggles <provisioning cluster>.rkeConfig.machineGlobalConfig[flannel-ipv6-masq]