[wayland] Drop support for legacy xdg-shell-v6 protocol
Summary:
xdg-shell stable has been around for quite a while. A quick analysis
showed that many distros ship GTK and Qt that support both xdg-shell
v6 and stable. Therefore, we can drop support for legacy v6 protocol.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: apol, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D28118