Survey and make changes needed for CMake 4.4 compatibility #996

CMake 4.4 (among other things) introduced a change in the way message(DEPRECATED) is handled—specifically whether the CMAKE_WARN_DEPRECATED and CMAKE_ERROR_DEPRECATED variables are honored. This behavior is controlled by a policy, [CMP0218](https://cmake.org/cmake/help/v4.4/policy/CMP0218.html). While Phlex does not use message(DEPRECATED), Cetmodules makes significant use thereof, and will need work to be compatible with CMake >=4.4. Currently the only indication is that non-fatal CMake warnings are generated indicating that CMP0218 is not set. Warnings can be suppressed by setting the appropriate CMAKE_POLICY variable to OLD (see #975), but going forward CMake Warnings with CMake 4.4.2 should be resolved, and the upper range of the cmake_minimum_required() call in Phlex' CMakeLists.txt should be increased to >=4.4.