The minimum CMake version for Qt6 is 3.16, so all the calls to
cmake_minimum_required() are updated here to enforce that
minimum. This will avoid any CMake version-related warnings
from Qt.
Avoid hard-coding Qt5 where the tests could now be using
Qt5 or Qt6.
Fixes: #22188
The QtAutogen/ManySources test generates a number of source, header, .ui and
.qrc files that get AUTOMOC, AUTOUIC and AUTORCC processed. This stresses the
concurrency framework in `cmQtAutoMocUic` and should reveal any issues
with that.