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.)