Configuration warning with updated CMake

See warnings at

https://lhcb-nightlies.web.cern.ch/nightly/lhcb-2024-patches/99/

CMake Deprecation Warning at CMakeLists.txt:199 (cmake_policy):
   The OLD behavior for policy CMP0104 will be removed from a future version
   of CMake.

This comes from here

https://gitlab.cern.ch/lhcb/Allen/-/blob/master/CMakeLists.txt?ref_type=heads#L199

this should be addressed so the warning is suppressed.