[release-1.27] fix: ignore Windows 2019 in ensureVMSSInPool for IPv6 backend pools only

[nilo19]

What type of PR is this?

/kind bug

What this PR does / why we need it:

doesn't try to add Windows 2019 VMSS to IPv6 backend pools because 2019 will never support IPv6 so it shouldn't be added.

Which issue(s) this PR fixes:

Fixes #6315

Special notes for your reviewer:

Does this PR introduce a user-facing change?

This only affects 1P, internal-to-Microsoft, customers. Windows 2019 with dual-stack is not possible otherwise.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nilo19

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[coveralls]

coverage: 77.911% (-0.01%) from 77.922%
when pulling 8b439f2 on nilo19:fix/cherry-pick-6316-1-27
into 95a67a1 on kubernetes-sigs:release-1.27.

[nilo19]

/retest

[nilo19]

/retest

[feiskyer]

/lgtm