replace golang.org/x/crypto/openpgp

[thepwagner]

Voucher relies on golang.org/x/crypto/openpgp , which is deprecated.

We should replace the usage of that library, or remove PGP as a signing option.

Related

• Deferred via https://github.com/grafeas/voucher/pull/72

[thepwagner]

FWIW I've personally used https://github.com/ProtonMail/go-crypto/tree/main/openpgp and it works dandy - drop-in replacement and additional features.

I forget the details of my usage but golang.org/x/crypto was missing some signature format that Debian(?) used.