"passphrase" encryption mode repos can not be created since borg 1.0.
back then, users were advised to switch existing repos of that type
to repokey mode using the "borg key migrate-to-repokey" command.
that command is still available in borg 1.0, 1.1 and 1.2, but not
any more in borg >= 1.3.
while we still might see the PassphraseKey.TYPE byte in old repos,
it is handled by the RepoKey code since borg 1.0.
