Summary:
Changing the creation of the nested window from unstable v5 to v6 in
order to be supported on more compositors and to use KWin's better
supported platform.

Targeting 5.12 as this will help to remove support for unstable v5
from KWayland.

Test Plan: Run a nested KWin/Wayland on KWin/Wayland, everything looked fine

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11806