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.