[SPI-2066]: replace deprecated GEANT4_BUILD_CXXSTD with CMAKE_CXX_STANDARD
There was a warning in cmake that GEANT4_BUILD_CXXSTD is deprecated, it is replaced with the cmake standard variable for the c++ standard
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There was a warning in cmake that GEANT4_BUILD_CXXSTD is deprecated, it is replaced with the cmake standard variable for the c++ standard