Update cmake_minimum_required to 3.16 in all CMake Qt example projects

We should update all our example projects to call cmake_minimum_required with version 3.16 rather than 3.14.

There are policies like https://cmake.org/cmake/help/latest/policy/CMP0091.html
which apply at the first call of project() which means that a subsequent call of cmake_minimum_required done in Qt6Config.cmake does not influence which msvc runtime library will be linked.

See https://codereview.qt-project.org/c/qt/qtbase/+/362442/1//COMMIT_MSG#17 for context.