[neon-notifications] kmail new version 18.12.3

[Neon CI]

https://build.neon.kde.org/job/watcher_release_kde_kmail/312/display/redirect

#<Debian::UScan::DEHS::Package:0x00005555ffaf1068 @name="kmail", @debian_uversion="18.12.2", @debian_mangled_uversion="18.12.2", @upstream_version="18.12.3", @upstream_url="http://download.kde.internal.neon.kde.org/stable/applications/18.12.3/src/kmail-18.12.3.tar.xz", @status="Newer version available">