Make printer column titles consistent across all types

That does, however, put us in an awkward spot since I think all other .status.startTimestamp fields should match, due to the existence of the creationTimestamp field. Ideally, we'd have clean mappings between the field name in the YAML and printer column, but I think due to historical reasons we're a little stuck.

I think we could put that on the docket for v2.0 to update those velero commands so we don't break any scripts.

Originally posted by @nrb in https://github.com/vmware-tanzu/velero/pull/2881#discussion_r496236736

In adding the additional printer columns for CRDs, it was noted that the columns for the get commands were named inconsistently from the fields being displayed, but also that they were inconsistent across types. For example, .status.startTimestamp is displayed under the column CREATED when running velero get backups but is displayed under the column STARTED when running velero get restores.

We should revisit each of these types and determine how they should be printed and ensure they are all consistent with each other.