Belatedly remove controller-manager IPv6DualStack feature gate

[danwinship]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

controller-manager still had an IPv6DualStack feature gate, which cloud-provider still checked. This feature went GA in 1.23.

Does this PR introduce a user-facing change?

The `IPv6DualStack` feature gate for external cloud providers was removed.
(The feature became GA in 1.23 and the gate was removed for all other
components several releases ago.) Action required: If you were still manually
enabling it you must stop now.

/cc @khenidak

[danwinship]

/sig network
/auto-cc

[aojea]

/lgtm
/assign @thockin @andrewsykim

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: f70ef2cf9ce40fc7ced95a1ed5e3ec9b5f19a077

[liggitt]

/lgtm
/approve

needs an action-required release note that the feature gate is removed, and open a PR to the dev-1.27 branch of the website to adjust https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates-removed/ and https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danwinship, liggitt

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Needs approval from an approver in each of these files:

• staging/src/k8s.io/cloud-provider/OWNERS [liggitt]
• staging/src/k8s.io/controller-manager/pkg/features/OWNERS [liggitt]

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[andrewsykim]

ref: #115204 (comment)

[danwinship]

needs an action-required release note that the feature gate is removed

done

and open a PR to the dev-1.27 branch of the website to adjust https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates-removed/ and https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/

I guess I didn't say this explicitly above, but the feature gate existed in several other components as well but was already removed. So it's already listed on the "feature-gates-removed" page, and not listed on the "feature-gates" page. There doesn't seem to be any precedent there about gates removed from different components at different times. (Since, obviously, that's not supposed to happen...) But it seems reasonable to just pretend it was removed everywhere in 1.25? No one is likely to be hurt by that...

[liggitt]

ah, I missed this was a secondary cleanup of an already removed gate... yeah... this seems fine

[fedebongio]

/triage accepted