Replace deprecated golang/x/crypto/openpgp

[NamelessOne91]

Related to: https://github.com/SUSE/spacewalk/issues/26645

As of golang/go#44226 openpgp support has been dropped from Go's standard and experimental libraries.

It would be better to replace it with something still maintained and more up-to-date, even though we have to deal with RPMs signed with OpenPGP V3 signatures (old standard).

To that end:

1. github.com/ProtonMail/go-crypto is a fork and drop-in replacement for golang/x/crypto, with openpgp support. However, they made quite a few changes and eventually dropped V3 signatures compatibility.
2. github.com/pgpkeys-eu/go-crypto is a soft fork of the above which provides also V3 signatures compatibility

[Bischoff]

I was not able to check all details of vendoring 🧌

[ktsamis]

I don't see anything obvious but I didn't check authors contributors etc, these docs. And for the code I'm not sure I'm qualified but nothing looks completely wrong