GnuPG 2.3.0 (2021-04-07) switched the default key algorithm to
ed25519/cv25519. Even though we're not currently using such a new
release, this is a good signal that we should start doing the same
for our artifact signing keys. Thankfully our current GPG version on
bridge can create them using the --expert option, so document the
slight changes to the required commands and update the example
output to more closely match its new behavior.
While we're here, the version we're using also autogenerates
revocation certificates. Take advantage of that to slightly simplify
our key generation instructions.
Change-Id: Ibb1c5ae8c540713e1c39d0000497c6b8b89b67c8
fb418c2d4a