Simplify text shown for (stable, removed) feature gates

[lmktfy]

Handle removed features differently. When a feature has graduated to stable and has since been removed, don't show the info box that we use for feature gates that Kubernetes still recognizes. Instead, show a simple paragraph explaining the situation.

/area web-development

The idea I have is to use the metadata we already have (removed) to decide what text to show to readers.

For previews, see #52632 (comment)

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[lmktfy]

Example of a feature gate not marked as removed, legacy means to specify it:
https://deploy-preview-52632--kubernetes-io-main-staging.netlify.app/docs/reference/access-authn-authz/authentication/#bootstrap-tokens

No explanatory text but still works.

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Example of a feature gate not marked as removed, recommended means to specify it, recent graduation:
https://deploy-preview-52632--kubernetes-io-main-staging.netlify.app/docs/reference/access-authn-authz/authentication/#using-authentication-configuration

This is a stable feature in Kubernetes, and has been since version 1.34. It was first available in the v1.29 release.

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Example of a feature gate marked as removed, recommended means to specify it:
https://deploy-preview-52632--kubernetes-io-main-staging.netlify.app/docs/concepts/architecture/nodes/#node-topology

This is a stable feature in Kubernetes, and has been since version 1.27. It was first available in the v1.16 release. You can no longer disable or opt of of this behavior.

I have deliberately picked an example that is a little stale.

Even if we make this this change, we should revise https://kubernetes.io/docs/concepts/architecture/nodes/#node-topology to remove ”If you have enabled the TopologyManager feature gate, then…”. I would also remove the feature gate shortcode from that section at the same time, as it's been stable for a while. However, this PR is not the place to fix that. We could use #52626 for that work, or tweak #52626 to leave in some feature gates where the removal is relatively recent.

Bear in mind that if we add metadata to record whether feature gates are locked, we can tweak the logic without needing to change the message. The statements in the docs would remain true.

[lmktfy]

Want different text? Feel free to suggest changes to it.

[lmktfy]

This text / template is easy to amend.

[tengqm]

There are some slight differences between the following states of a feature gate:

• GA'ed but not "locked": you can specify the gate, and you can opt out;
• GA'ed and "locked": you can specify the gate, but you cannot opt out;
• Removed: you can NOT specify the gate, it is completely gone, no longer recognized as a valid gate.

For gates that have been removed, we may want to drop the anchors instead of carry them on forever.

[lmktfy]

There are some slight differences between the following states of a feature gate:

• GA'ed but not "locked": you can specify the gate, and you can opt out;
• GA'ed and "locked": you can specify the gate, but you cannot opt out;
• Removed: you can NOT specify the gate, it is completely gone, no longer recognized as a valid gate.

I agree. And I hope that a future PR continues our improvement journey.

For gates that have been removed, we may want to drop the anchors instead of carry them on forever.

Anchors?

If you mean shortcodes, then removing the shortcodes can still happen. I propose retaining the shortcode for a number of releases after it has been removed, or since it became stable.