GCE: enable NAT64 for IPv6-only clusters

[shreyaabaranwal]

What this does

GCE pods on an IPv6-only cluster currently have no way to reach
IPv4-only destinations — there's just no NAT64 path for them. AWS
already solved this with a 64:ff9b::/96 route (see
pkg/model/awsmodel/network.go), so this PR brings GCE up to speed by
enabling NAT64 on the Cloud NAT router whenever the cluster is
IPv6-only (spec.IsIPv6Only()).

Scope

Good news: this turned out smaller than I expected, because GCE's
Cloud NAT already has built-in NAT64 support — it was just never
wired up in kOps.

Per the discussion on #17840, this PR sticks to the part apeabody
flagged as the priority: pod/service networking goes IPv6-only,
while the VMs and subnets stay comfortably dual-stack
(stackType=IPV4_IPV6) for now. Making the VMs/subnets themselves
IPv6-only is a bigger lift and is left for a follow-up PR.

Changes

• upup/pkg/fi/cloudup/gcetasks/router.go — adds a
  SourceSubnetworkIPRangesToNAT64 field to the Router task, wired
  through Find, RenderGCE, and RenderTerraform. (Also added a
  small nil-guard here so non-IPv6 clusters don't end up with a
  permanent phantom diff on every kops update cluster — found that
  one the hard way during testing.)
• pkg/model/gcemodel/network.go — sets NAT64 to
  ALL_IPV6_SUBNETWORKS on the Cloud NAT router when
  b.IsIPv6Only() is true. Five lines, does its job.
• pkg/model/gcemodel/network_test.go — new table-driven test
  covering both the IPv6-only and non-IPv6-only paths, since there
  wasn't an existing unit test file for network.go to extend.

Testing

• go build ./upup/pkg/fi/cloudup/gcetasks/... ./pkg/model/gcemodel/... ✅
• go test ./upup/pkg/fi/cloudup/gcetasks/... ./pkg/model/gcemodel/... ✅

This is my first contribution to kOps, so very open to feedback on
approach, naming, or anything I might've missed!

/kind feature
Part of #17840

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[shreyaabaranwal]

Opened a first PR for this: #18520

Scope: enables NAT64 on the GCE Cloud NAT router when the cluster
is IPv6-only, so pods can reach IPv4-only destinations — VMs and
subnets stay dual-stack as you suggested. Happy to adjust based
on review!