Reland "[CMake] Bumps minimum version to 3.20.0."

All build bots should be updated now.

This reverts commit 44d38022ab29a3156349602733b3459df5beef93.

GitOrigin-RevId: ffb807ab5375b3f78df198dc5d4302b3b552242f
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 89f08b8..1e273d4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.13.4)
+cmake_minimum_required(VERSION 3.20.0)
 
 project( libclc VERSION 0.2.0 LANGUAGES CXX C)