snap: Don't use deprecated API
Replace the use of snapd_client_list_sync with snapd_client_get_snaps_sync, snapd_client_list_one_sync with snapd_client_get_snap_sync, and gs_app_set_developer_name with snapd_snap_get_publisher_username.
Replace the use of snapd_client_list_sync with snapd_client_get_snaps_sync, snapd_client_list_one_sync with snapd_client_get_snap_sync, and gs_app_set_developer_name with snapd_snap_get_publisher_username.