Build: Bump CMake mini version to 3.21

CMake 3.15 is a bit out-of-date.
Bump mini version to 3.21 to take advantage of newer version.

Change-Id: I1162aebbfb50507925d6aa86546dcfc41b7aa4de
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index dee167a..3516487 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -6,7 +6,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 #
 #-------------------------------------------------------------------------------
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.21)
 
 SET(CMAKE_SYSTEM_NAME Generic)