cmake_minimum_required 2.x is too old

[tkittel]

https://github.com/McStasMcXtrace/McCode/blob/2f807a909b79f3105a770c986cf1a622b9afc212/mcstas/CMakeLists.txt#L1C1-L2C1

Causes warnings in conda builds (where CMake is brand new) like:

CMake Deprecation Warning at mcstas/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

We should probably update all of them to the 3.17 that we use in the top-level CMakeLists.txt.