Ignore "kwin4" effect plugins
In a self compiled KF6, effects from KF5 were shown in the KCM, with the same name and general look as their KF6 relatives. There was nothing to explicitly mark the KF5 effects as incompatible with KF6, and AFAIU they actually (mostly? for now?) aren't, but having the two entries per effect was of course quite confusing.
There is deduplication logic in KPluginLoader, but that only works for plugins with the same name / ID, which does nothing here because the KF5 effects start with "kwin4_effect_" and the KF6 effects don't. That same fact can be used to filter out the KF5 effect, which is what this patch does.