packagekit: Bump PackageKit dependency to 1.2.5 and drop old fallbacks

1.2.5 is available in Fedora 36 onwards, and in Debian Bookworm onwards,
which should be sufficient.

Dropping the fallback code (particularly the `gs_systemd_call_*()`
functions) should make upcoming changes simpler.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1472