cmake 4.2.3 dropped support for cmake_minimum_required below 3.5

[bgemmill]

I upgraded to cmake 4.2.3 from the kitware repository today to see about building on ubuntu 26.04 when it comes out, and it looks like cmake no longer supports cmake_minimum_required below 3.5.

When compiling libzmq with cmake, I get errors like:

CMake Error at CMakeLists.txt:7 (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.

Can the supported version of cmake please be bumped up so this continues building?