Raise required CMake version to 3.16
This was discussed in https://mail.kde.org/pipermail/plasma-devel/2021-February/119329.html and https://mail.kde.org/pipermail/distributions/2021-February/000943.html and nobody objected.
This was discussed in https://mail.kde.org/pipermail/plasma-devel/2021-February/119329.html and https://mail.kde.org/pipermail/distributions/2021-February/000943.html and nobody objected.