Move away from golang openpgp

[MatthewJohn]

According to the docs: https://pkg.go.dev/golang.org/x/crypto/openpgp, the library has been deprecated:

Deprecated: this package is unmaintained except for security fixes. New applications should consider a more focused, modern alternative to OpenPGP for their specific task. If you are required to interoperate with OpenPGP systems and need a maintained package, consider a community fork. See https://golang.org/issue/44226. 

It seems that quite a few have suggested/have moved to https://github.com/ProtonMail/gopenpgp
This arose whilst trying to write some tests: c25d831

[yermulnik]

Just cross-posting for visibility: #435 (comment)

I wonder if these tests can immediately start using Proton Mail's library to see if it resolves the issues

I'd say go ahead with it.

[MatthewJohn]

Re-opening as gopenpgp was only used for some mocking tests - the original use of openpgp is still in use