Increase CMake required version 3.5 -> 3.20

Commit 777e0b2 introduced cmake_path
which has been added to Cmake in 3.20 onlyi [1], so this should only make
the error message more explicit.

[1] https://cmake.org/cmake/help/git-master/command/cmake_path.html

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.20 FATAL_ERROR)
 
 if (POLICY CMP0075)
   cmake_policy(SET CMP0075 NEW)