[neon-notifications] plymouth-kcm new version 5.15.4

[Neon CI]

https://build.neon.kde.org/job/watcher_release_kde_plymouth-kcm/344/display/redirect

#<Debian::UScan::DEHS::Package:0x0000561b35c3b9b8 @name="plymouth-kcm", @debian_uversion="5.15.3", @debian_mangled_uversion="5.15.3", @upstream_version="5.15.4", @upstream_url="https://download.kde.internal.neon.kde.org/stable/plasma/5.15.4/plymouth-kcm-5.15.4.tar.xz", @status="Newer version available">