[neon-notifications] Releasing: knotes - 21.04.0

[Neon CI]

https://build.neon.kde.org/job/watcher_release_kde_knotes/1104/display/redirect

#<Debian::UScan::DEHS::Package:0x000055c088d35ba0 @name="knotes", @debian_uversion="20.12.3", @debian_mangled_uversion="20.12.3", @upstream_version="21.04.0", @upstream_url="http://download.kde.internal.neon.kde.org/stable/release-service/21.04.0/src/knotes-21.04.0.tar.xz", @status="Newer version available">