Update minimum cmake version

[PrasadBabarendaGamage]

Change minimum cmake version to v3.20. (>3.16?)

Error when building on cmake v3.15.17: cmake --build .

[ 88%] Creating config.
-- Attempting to use gnu compilers.
CMake Error at /usr/local/share/cmake-3.15/Modules/FindPackageHandleStandardArgs.cmake:195 (message):
Unknown keywords given to FIND_PACKAGE_HANDLE_STANDARD_ARGS():
"REASON_FAILURE_MESSAGE"
Call Stack (most recent call first):
/home/clin864/opt/OpenCMISS/bn363773/opencmiss/install/share/cmake/Modules/FindPython/Support.cmake:2984 (find_package_handle_standard_args)
/home/clin864/opt/OpenCMISS/bn363773/opencmiss/install/share/cmake/Modules/FindPython.cmake:415 (include)
CMakeLists.txt:140 (find_package)
-- Configuring incomplete, errors occurred!
See also "/home/clin864/opt/OpenCMISS/bn363773/opencmiss/build/manage/release/config/Release/CMakeFiles/CMakeOutput.log".
CMakeFiles/manage_config.dir/build.make:78: recipe for target 'config/Release/stamp/manage_config-create' failed
make[5]: *** [config/Release/stamp/manage_config-create] Error 1
CMakeFiles/Makefile2:164: recipe for target 'CMakeFiles/manage_config.dir/all' failed
make[4]: *** [CMakeFiles/manage_config.dir/all] Error 2
Makefile:94: recipe for target 'all' failed
make[3]: *** [all] Error 2
CMakeFiles/setup_manage.dir/build.make:112: recipe for target 'setup_manage-prefix/src/setup_manage-stamp/setup_manage-build' failed
make[2]: *** [setup_manage-prefix/src/setup_manage-stamp/setup_manage-build] Error 2
CMakeFiles/Makefile2:136: recipe for target 'CMakeFiles/setup_manage.dir/all' failed
make[1]: *** [CMakeFiles/setup_manage.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

[vraj004]

I think this issue is related to issue covered here: OpenCMISS/iron#192