Bump cmake requirement
see https://bugs.gentoo.org/965034
--- a/stubby/CMakeLists.txt
+++ b/stubby/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)