CMake: set cmake_minimum_required to 3.10

[nim65s]

Hi again,

To be more future-proof, I thinks this should be updated, as we currently have this warning:

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

For reference, the current oldest maintained debian already has CMake 3.18, from 2020. CMake 3.10 is from 2018.

[nim65s]

rebased, and bumped googletest tag to fix ci:

-- FETCHCONTENT_SOURCE_DIR_GOOGLETEST: /home/runner/work/EigenRand/EigenRand/googletest
CMake Error at googletest/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.