Change minimal required cmake version to 3.16 (#1388)

CMakeLists.txt

@@ -1,6 +1,6 @@
 # Author: Kang Lin <[email protected]>
 
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.16)
 
 project(coturn)
 set(CMAKE_INCLUDE_CURRENT_DIR ON)