Require CMake 3.16
Fixes CMake warning:
/usr/share/ECM/modules/ECMFindModuleHelpers.cmake:112: warning: Your project should require at least CMake 3.16.0 to use FindKF5.cmake /usr/share/ECM/find-modules/FindKF5.cmake:30 (ecm_find_package_version_check) CMakeLists.txt:30 (find_package)