Move asset copying out of apply_cluster

[johngmyers]

Removes the assets phase, moving the functionality to the new kops get assets --copy flag.

[rifelpet]

I'm wondering if we should announce the deprecation of the assets phase in the 1.21 release notes.

This will also need mentioning in the 1.22 release notes, and ideally have documentation that walks through the process of how to use get assets --copy but that can be in a followup PR.

[johngmyers]

There's nothing a user could do with such a deprecation notice. The feature isn't going away; the command to invoke it is just changing.

My documentation PR #11654 is pending review, so I can't update it in this PR.

[rifelpet]

True, but I think the 1.22 release notes should still mention the move of functionality. I'm fine with that being in a followup PR

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rifelpet

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rifelpet]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment