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

Some build bots have not been updated to the new minimal CMake version.
Reverting for now and ping the buildbot owners.

This reverts commit 44c6b905f8527635e49bb3ea97dea315f92d38ec.

GitOrigin-RevId: 92523a35a827539db8557bbc3ecab7f9ea3f6ade
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1e273d4..89f08b8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.20.0)
+cmake_minimum_required(VERSION 3.13.4)
 
 project( libclc VERSION 0.2.0 LANGUAGES CXX C)