Replace `extensions/v1beta1` `Ingress` in documentation

[jsoref]

There are many instances of extensions/v1beta1 Ingress in this repository, e.g.:
https://github.com/kubernetes/ingress-gce/blob/051cfde145f34c8721e39df778e262fb4a256c67/LIMITATIONS.md#latency

Per https://endoflife.date/google-kubernetes-engine, it appears that 1.21 will expire at the end of the year. That will be the last version of GKE that supported extensions/v1beta1 Ingress which was deprecated in 1.14 -- https://kubernetes.io/docs/reference/using-api/deprecation-guide/#ingress-v122

[kundan2707]

/assign

[spencerhance]

Thanks @jsoref and @kundan2707 !

This change makes sense to me! We would definitely accept a PR for this fix.

[k8s-triage-robot]

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[jsoref]

Sigh

[kundan2707]

/remove-lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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/lifecycle rotten

[jsoref]

This is not stale, and it really matters.

This repository is stale.