Bring qt6_finalize_project into shape for 6.3

Add a strong global warning in qt_add_executable()/find_package(Qt6Core) that the user should call qt6_finalize_project, when targeting Android and CMake 3.18.
https://codereview.qt-project.org/c/qt/qtbase/+/365396/23/src/corelib/Qt6AndroidMacros.cmake#428

Add an option to disable the global warning.
QT_NO_WARN_BUILD_TREE_APK_DEPS disables the warning

Add a strong warning in qt documentation to use latest CMake, otherwise users would need to call this function at least to cover the android specific implementation

Document what the function does. Mark as TP.

Note: Qt Creator wizard will not generate this, because the function might go away after we bump min CMake requirement to 3.18.