[NEW FEATURE ] post-RFC4880 version compatibility

Although strictly speaking OpenPGP clients MUST cleanly ignore unknown future versions, this is not always obeyed. Concerns have been raised due to the lack of correct handling of future key versions in the wild (see the "Version 23" tests at https://tests.sequoia-pgp.org/#Perturbed_certificates). It has been suggested that we need a modifier variable to indicate the key versions to be returned, and have it default to {3,4} or {4}.