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CMake 3.15 is a bit out-of-date.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/docs/CMakeLists.txt b/docs/CMakeLists.txt
index eb17631..7b8eef9 100644
--- a/docs/CMakeLists.txt
+++ b/docs/CMakeLists.txt
@@ -5,7 +5,7 @@
 #
 #-------------------------------------------------------------------------------
 
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.21)
 
 add_custom_target(docs)