KEP template: remove checkboxes

[pohly]

The check lists are useful. The check boxes are not: presumably ticking them off is meant to document that authors and reviewers have looked at the list. But that only works once. In following releases, the box is ticked off whether the related check list item was checked again or not.

These boxes cause confusion repeatedly because some of them need to be ticked off before the list item is actually done (PRR review/approval). Others would have to be updated later in a second PR, which causes extra work for approvers.

In practice, most of the KEPs don't seem to bother with ticking these boxes:

$ git grep -e '- \[[xX]\]' keps | wc -l
3611
$ git grep -e '- \[ \]' keps | wc -l
3821

That may be legitimate in a few cases (GA), but in most cases it just shows that no-one cared. Let's remove them to simplify the KEP process.

Previously discussed in https://kubernetes.slack.com/archives/C1L57L91V/p1745311988152919

/assign @kikisdeliveryservice

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pohly
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[kikisdeliveryservice]

Thanks for the conversation on the checklist. We will be refining both the list and instructions to address some confusion and painpoints raised in discussion. However, we will be keeping some iteration of a checklist for authors to use.

/hold