Add @drbenmorgan as maintainer for core HEP packages (#15264)

var/spack/repos/builtin/packages/clhep/package.py

var/spack/repos/builtin/packages/g4abla/package.py

var/spack/repos/builtin/packages/g4emlow/package.py

var/spack/repos/builtin/packages/g4ensdfstate/package.py

var/spack/repos/builtin/packages/g4ndl/package.py

var/spack/repos/builtin/packages/g4neutronxs/package.py

...ack/repos/builtin/packages/g4photonevaporation/package.py

var/spack/repos/builtin/packages/g4pii/package.py

...pack/repos/builtin/packages/g4radioactivedecay/package.py

var/spack/repos/builtin/packages/g4realsurface/package.py

var/spack/repos/builtin/packages/g4saiddata/package.py

var/spack/repos/builtin/packages/g4tendl/package.py

var/spack/repos/builtin/packages/geant4-data/package.py

var/spack/repos/builtin/packages/geant4/package.py

var/spack/repos/builtin/packages/root/package.py