docs: maintainer-pgp-guide: update for latest gnupg defaults

docs: maintainer-pgp-guide: update for latest gnupg defaults

It is finally becoming increasingly rare to find a distribution that
still ships with gnupg-1.x, so remove the last vestiges of "gpg" vs
"gpg2" from documentation.

Similarly, starting with GnuPG 2.2 and above, the default --gen-key
operation creates ed25519/cv25519 keypairs, so update all example
command outputs to use that combination instead of rsa2048.

Lastly, add a few wording tweaks and remove links that lead to stale
information (e.g. hardware tokens overview from 2017).

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20221220-docs-pgp-guide-v1-1-9b0c0bf974fb@linuxfoundation.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>