Build: Bump cmake minimum version to 3.21

Take advantage of newer verion to simplfy build script implementations.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I514844b9de3c847c81665c090639a80a2ea9d360
diff --git a/tests_reg/CMakeLists.txt b/tests_reg/CMakeLists.txt
index 619cea9..b7821e8 100644
--- a/tests_reg/CMakeLists.txt
+++ b/tests_reg/CMakeLists.txt
@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 #
 #-------------------------------------------------------------------------------
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.21)
 
 # CMake 3.21 and above requests projects to specify cpu/arch compile and link flags explicitly in
 # Armclang. Link: https://cmake.org/cmake/help/latest/policy/CMP0123.html