Finish deprecating WId-based KWindowEffects API
Open, Needs TriagePublic

Description

https://invent.kde.org/frameworks/kwindowsystem/-/merge_requests/21 deprecates the public WId-based KWindowEffects API but not the private API in KWindowEffectsPrivate because it will break Plasma. KWaylandIntegration and KWin provide their own KWindowSystem platform plugins.

Once KF6 is branched, the leftover WId-based API in KWindowEffectsPrivate needs to be replaced with a QWindow-based API.