[TF-M] [RFC] Bump minimum CMake version required

Hi all,

The current minimum CMake version specified in TF-M is 3.15. It is quite out-of-date. I’d like to propose to increase minimum CMake version to 3.21.

CMake 3.21 can bring in some benefits, compared to CMake 3.15

* NEW behavior of several CMake policies, such as CMP0123 for Armclang. * After build split, TF-M secure build installs several build scripts. When NS build includes those build scripts, `CMAKE_CURRENT_FUNCTION_LIST_FILE/DIR` can make it easier to handle source code paths.

CMake 3.21 was released 2 years ago. It should be stable enough now.

May I know if you have any feedback or different opinions? Thank you.

Best regards, Hu Ziji