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/app_broker/CMakeLists.txt b/app_broker/CMakeLists.txt
index eaf4b3f..f24e612 100644
--- a/app_broker/CMakeLists.txt
+++ b/app_broker/CMakeLists.txt
@@ -8,7 +8,7 @@
 # This CMake script used in a split build only. In the legacy 'sinlge' build
 # this file is ignoed. Please don't be confused.
 #
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.21)
 
 # This is important to add this SPE subdirectory at first as it
 # brings configuration from TF-M build