switch browser extension to OpenPGP v5

[tomholub]

Not ready yet, but hopefully in a few months it will be

[tomholub]

This will be a massive effort, I think. We may wait until later in the year to do this.

[tomholub]

Maybe late October.

[IvanPizhenko]

@tomholub There's PR #3955 initiated by dependabot

[limonte]

@IvanPizhenko I guess our openpgpjs-custom.ts middleware is not compatible out of the box with the proposed OpenPGP.js upgrade

Cannot read property 'Key' of undefined
  › Object.<anonymous> (test/source/core/crypto/pgp/openpgpjs-custom.ts:36:12)

Originally posted by @rrrooommmaaa in #3955 (comment)