CMake: Bump all cmake_minimum_required calls to 3.16

Needed for subsequent change that will check and error out if the
version is lower than 3.16. We do that to ensure all policies
introduced by CMake up to version 3.16 have their behavior set to
NEW.

Pick-to: 6.2
Task-number: QTBUG-95018
Change-Id: I83afb14f10dc0a8651625950ad6a873382db010e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

CMakeLists.txt

 # Generated from qttranslations.pro.
 
-cmake_minimum_required(VERSION 3.15.0)
+cmake_minimum_required(VERSION 3.16)
 
 include(.cmake.conf)
 project(QtTranslations # special case