upgrade CMake in dockerfile-cli (fixes #6420)

[narumiruna]

$ docker build -t lightgbm-cli -f dockerfile-cli .

14.05 CMake Error at CMakeLists.txt:30 (cmake_minimum_required):
14.05   CMake 3.18 or higher is required.  You are running version 3.16.3

This PR upgrades CMake to the newest version.

[jameslamb]

Thanks very much! This looks good to me. I think it'll fix #6420, so I've added that to the description.

In the near future, we'll added automated tests on these Dockerfiles to prevent issues like this from arising in the future.

[jameslamb]

Thanks very much, we'd love to have you come contribute more in the future!