snap: update for Qt6
this MR
- update snap to core22
- fix snap build for Qt6
- add wayland support
It's currently a transitioning period for kde/qt6/ubuntu (kf6 is just released and not available in ubuntu 22, kf6 snap package is not available yet and core24 is not yet available), so a lot a integration that is usually provided by the kde-neon have been done manually.
depends on docker-images!288 (merged) for the docker image, I'll update the gitlab-ci.yml snap image once its merged
depends on !4783 (merged) for the Qt6 part, only review the last 4 commits.