cmake: depend on CMake 3.16

Change-Id: I5cf72bbbdc5b5c6687c353fe67923a1fa1d1d0fa
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 21d7b69..9b42175 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.20)
+cmake_minimum_required(VERSION 3.16)
 project(dhtnet)
 
 set(CMAKE_CXX_STANDARD 17)