mimalloc cmake 3.0 compatibility with cmake 4

[krulf]

When building with cmake 4 it throws an error on configuring mimalloc as it has minimum cmake versions set to 3.0 and version 4 has dropped support for compatability older than 3.5.

Updating extlib/mimalloc/CMakeLists.txt to minimum version 3.5 or adding flag -DCMAKE_POLICY_VERSION_MINIMUM=3.5 both seem to work.

https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html

[daanx]

It seems the current required minimum is 3.18 -- that should work right?