bionic-amd64-tf-m-build: Upgrade CMake to 3.21

CMake 3.15 is out-of-date. Bump CMake version to 3.21.

Change-Id: Ifcecdf610e4cc1ea56bc4ae5130b809ee3d3066f
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed