Backport: Let CMake handle setting of CMP0135 (VECGEOM-622)
Simple backport of the fixes from !1083 (merged) to the v1-patches
branch:
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.