The source project of this merge request has been removed.
Set KDE_SESSION_VERSION to 6
Since the master branch is based on Qt6/KF6 and will become Plasma 6, set KDE_SESSION_VERSION to "6".
This will allow other applications to detect they're running in a (preview of a) Plasma 6 session and react accordingly. (E.g. LibreOffice can use this to choose to use its upcoming kf6 integration by default instead of the kf5 one.)