Add example showing how to upgrade k8s version

[prasadkatti]

Stop-gap solution for #6122 until we have a separate page which discusses how to upgrade a minikube cluster.

[k8s-ci-robot]

Hi @prasadkatti. Thanks for your PR.

I'm waiting for a kubernetes 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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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: prasadkatti
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[minikube-bot]

Can one of the admins verify this patch?

[prasadkatti]

@tstromberg - I can work on #6122 as well, I think. But I will need help.