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.

[linux-foundation-easycla[bot]]

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• :white_check_mark: login: CathalC2011 / name: Cathal (42ba7f1fa75df087f8313fdf0838797e56259c43)

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[istio-testing]

Hi @CathalC2011. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ericvn]

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