replace golang.org/x/crypto/openpgp

Voucher relies on golang.org/x/crypto/openpgp , which [is deprecated](https://golang.org/issue/44226).

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

Start by locating Voucher's use of golang.org/x/crypto/openpgp and read the deferred discussion in pull request 72. Determine whether the project will replace the library or remove PGP signing; done means the deprecated dependency is no longer used and the selected signing behavior is covered by the project's existing checks.

Written by the indexing model from the issue text.