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

The owner of the last two failing buildbots updated CMake.

This reverts commit e8e8707b4aa6e4cc04c0cffb2de01d2de71165fc.

GitOrigin-RevId: 65429b9af6a2c99d340ab2dcddd41dab201f399c
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0eda126..6a21544 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)