Compatibility issue with CMake 4.0

[PhilMiller]

Our embedded copy of pybind11 and other bits declare minimum CMake versions below 3.5. That's become a problem in CMake 4.0, which removes compatibility further back than that. Thus, the configuration step fails:

CMake Error at extern/pybind11/CMakeLists.txt:8 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from 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.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

Our copy of GoogleTest also has this issue.

To actually not get any warnings, the declared acceptable version of CMake has to extend to at least 3.10, since 3.5..3.9 support is deprecated and planned for removal as well.