Add --etc-upgrade flag to kubeadm upgrade plan

[tiesmaster]

What would you like to be added?

Add the flag --etc-upgrade that is present on kubeadm upgrade apply also to the subcommand kubeadm upgrade plan. If you specify this, then the etcd upgrade versions won't be calculcated, and it states no upgrade version for etcd (similar to like how plan won't show a version when it detects that external etcd is in use).

Why is this needed?

To align the flags between the subcommands plan, and apply. There are already a bunch of flags to control the target version (--allow-*-upgrades), and they exist on both subcommands. On the apply you also have the flag to disable upgrading etcd, but you don't have that on plan, so this won't allow you to see what plan exactly would upgrade. Adding this flag will allow the user to exactly see what would happen when running the apply subcommand subsequent to the plan invocation.

/sig cluster-lifecycle
/cc @neolit123

Slack discussion: https://kubernetes.slack.com/archives/C2P1JHS2E/p1738258436793859

[k8s-ci-robot]

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[neolit123]

/transfer kubeadm

[neolit123]

there is also an configuration api (--config) where this is present for apply.