add note prior to interactive confirm of kubeadm upgrade apply

[jipperinbham]

What this PR does / why we need it:
informs users when running kubeadm upgrade apply in interactive mode to first copy the existing config files in the event they are wanting to diff the changes between versions

Which issue(s) this PR fixes:
Fixes kubernetes/kubeadm#489

Special notes for your reviewer:

Release note:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jipperinbham
We suggest the following additional approver: luxas

Assign the PR to them by writing /assign @luxas in a comment when ready.

Associated issue: #489

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[dims]

/ok-to-test

[k8s-ci-robot]

@jipperinbham: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-unit	73108a3	link	/test pull-kubernetes-unit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[luxas]

Hi @jipperinbham!
Thanks for this contribution :)

What is the rationale for adding this information?
This is why we added --dry-run, don't you think that is enough for this purpose?
(Feel free to disagree with me :P)

[errordeveloper]

@jipperinbham is this because --dry-run doesn't actually create files that you can diff directly? In which case, it still seems a little niche to print to the standard output.

[fejta-bot]

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[jipperinbham]

much better solution was proposed and implemented, #63930