import: rename keyring extension from .gpg to .pgp and add OBS key for system:systemd...

import: rename keyring extension from .gpg to .pgp and add OBS key for system:systemd (#37927)

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.