Bump minimum cmake version

[jschueller]

Avoids this warning:
Compatibility with CMake < 3.10 will be removed from a future version of CMake.
cmake 3.13 should be still pretty old to allow for maximum compatibility

/cc @barche

[yuyichao]

Note that the preferred way to handle this is to use a version range like 3.5...4.0. This actually works on old cmake version as well and was the intention of this syntax. See https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html

[yuyichao]

(Not that I think dropping 3.12 support should be a problem, but a max version can/should be added to improve compatibility going forward.)