Secret checkpoints written before #206 can no longer be resumed

[vishr]

Problem

#206 dropped support for the onebox.run/secret-checkpoint/v1alpha1 schema. A ob secrets push interrupted before the upgrade leaves a v1alpha1 checkpoint in the release, and every later load fails with secret checkpoint schema ... is not supported, so neither recovery nor a new secrets push can proceed.

Fix

Add to guides/upgrade-to-onebox-names: finish or abort any interrupted ob secrets push before upgrading, or delete the stale checkpoint file after confirming the secret generation it describes. Optionally make the error message name the file and this remedy.