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CMake 3.15 is a bit out-of-date.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>
29 files changed