[VECGEOM-622] Let CMake handle setting of CMP0135
MR !1045 (merged) addressed the problem of CMP0135 policy warnings from the use of ExternalProject for builtin VecCore, but the solution is only compatible with CMake 3.24 and newer.
Update fix by removing explicit use of DOWNLOAD_TIMESTAMP and only relying on cmake_minimum_required with a version range to set the policy appropriately.
Closes VECGEOM-622
Will require backporting to v1-patches
and surface_model