Correct Istio Upgrade version step message

[cathal-c]

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The existing message implies that upgrading two minor versions is supported, and indeed this has historically been the case, at least anecdotally.

The Istio 1.17 control plane however, is not compatible with Istio 1.15 proxies, breaking that soft rule. That should be reflected here, to prevent users from incorrectly jumping multiple minor versions.

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• ✅ login: CathalC2011 / name: Cathal (42ba7f1)

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Hi @CathalC2011. Thanks for your PR.

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[ericvn]

Would you point to where this is discussed. Is this maybe related to this istio/istio#43785? I believe there was an issue upgrading to 1.17.0 and 1.17.1 but it was fixed in 1.17.2.

[dhawton]

Suggested change

	Upgrading across more than one minor version (e.g., `1.15.x` to `1.17.x`) in one step is not officially tested or recommended.
	Upgrading across more than two minor version (e.g., `1.15.x` to `1.17.x`) in one step is not officially tested or recommended. Upgrades that skip a minor version should only be done when using [Canary Upgrades](https://istio.io/latest/docs/setup/upgrade/canary/).

Per John's guidance but still clarify the statement.

[linsun]

+1 should be supported for canary

[ericvn]

We do test some upgrade testing across two versions although I need to verify what it's doing: https://github.com/istio/istio/tree/master/tests/integration/helm/upgrade

[linsun]

Should we say - Upgrading across more than two minor version (e.g., 1.15.x to 1.17.x) in one step is not officially tested or recommended for in place upgrade.

[dhawton]

Personally would like to specifically call out Canary Upgrades and link to doc to help guide toward a set procedure that is better practice.

[Arhell]

/ok-to-test

[craigbox]

Saying "Upgrading across" implies skipping a version, and the original line read "Upgrading across two minor versions (e.g. 1.6 to 1.9)".

I think we need to clarify the statement a little further.