Once we flipped the switch to Qt 6, we should add a qute://warning page for Qt 5 users if autodetection was used and Qt 6 was not found. This will help for the (probably somewhat common) situation where someone uses mkvenv.py and does a git pull, but does not regenerate the environment (thus not switching from Qt 5 to Qt 6 even if we make it the default for mkvenv.py to use).