Use org.kde.khelpcenter as desktop id (and appstream id)
Most places use "khelpcenter" instead of "Help" to refer to the appliction, the string "org.kde.Help" is only ever used for the desktop file. Also does KDBusService register on D-Bus based on the set application name, thus org.kde.khelpcenter, not org.kde.Help, which cannot be easily adapted. There are currently two known consumers which try to find khelpcenter using KService::serviceByDesktopName("org.kde.help"), though seems that broke recently due to KService becoming case-sensitive. As both consumers (KGpg, Konqueror) are part of KDE Gear, they can be adapted in sync and fixed to directly use the new name.