Check for and notify of available OS upgrade

[cassidyjames]

Marius noted the Dameon might not need to "check" for OS upgrades; it could be hard-coded and we'd wait to release that update to the daemon until it's been well-tested

[meisenzahl]

@Conan-Kudo mentioned on Slack that Pk.Client.get_distro_upgrades is not used because there is no distro-agnostic way to discover OS updates.

GNOME Software uses this mechanism: https://gitlab.gnome.org/GNOME/gnome-software/-/blob/main/plugins/fedora-pkgdb-collections/gs-plugin-fedora-pkgdb-collections.c#L28

So it is not hard-coded in GNOME Software. But the information if an OS upgrade is available is provided via an API.