simplify cmake version checks

[ximex]

see: https://cmake.org/cmake/help/latest/command/if.html#version-comparisons

[past-due]

VERSION_GREATER_EQUAL is not available in CMake < v3.7, which we (currently) must support for some platforms.

[ximex]

Here cmake 3.10+ is mentioned
https://github.com/Warzone2100/warzone2100#windows-using-msvc
and here it's already used. this is why i thought this version requirement is no problem
https://github.com/Warzone2100/warzone2100/blob/master/configure_mac.cmake#L241

[ximex]

i see we still build for ubuntu 16.04 that has an older cmake version. do we really need ubuntu 16.04 builds? support ended April 2021

[past-due]

Yes, for Windows and macOS we recommend newer versions.

For other platforms (ex. Linux / BSDs) we currently only require CMake 3.5+, which is why we set the main requirement as such here:

https://github.com/Warzone2100/warzone2100/blob/master/CMakeLists.txt#L1