The format is not specific to gpg, it's defined by the pgp
protocols. Lintian even warns about it:
systemd-container: openpgp-file-has-implementation-specific-extension [usr/lib/systemd/import-pubring.gpg]
Rename it accordingly, as new pgp implementations are slowly becoming
more popular.
Keep using /etc/systemd/import-pubring.gpg for backward compatibility.
This also replaces the Fedora download example with another one from
Ubuntu, since Fedora's images these days no longer qualify as DDIs, they
have no distinctive partition type UUIDs set for multiple of their
partitions, hence the images cannot be booted. A bit sad. Let's provide
a command that just works in its place.
