Replace Deprecated OpenPGP Package

[tri-adam]

The golang.org/x/crypto/openpgp package is now deprecated (ref). We should consider moving to a community-maintained fork such as github.com/ProtonMail/go-crypto, probably in lockstep with github.com/sylabs/sif/v2 (see sylabs/sif#129).

[dtrudg]

Agreed this makes sense. The ProtonMail fork has come up before w.r.t. investigating whether the customization Singularity uses could be ported: apptainer/singularity#5407

The note there was that the impediment to this was breaking SIF... so 🤞 this might be relatively straightforward here.