Downgrade minimum cmake version to 3.16

Which matches SDL3 and ubuntu 20.04.
I checked the release notes, there were no relevant changed in 3.17 and 3.18.

CMakeLists.txt

@@ -23,7 +23,7 @@ if(${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
 	message(FATAL_ERROR "Prevented in-tree build.")
 endif()
 
-cmake_minimum_required(VERSION 3.18)
+cmake_minimum_required(VERSION 3.16)
 
 project(love)