Update to Geant4 10.7.3

For the record, this is the procedure I used:

git clone ssh://git@gitlab.cern.ch:7999/lhcb/Geant4.git
cd Geant4
git fetch
git fetch https://gitlab.cern.ch/geant4/geant4.git v10.7.3
git checkout -b upstream-v10.7.3 FETCH_HEAD
git fetch https://gitlab.cern.ch/geant4/geant4.git v10.6.2
git merge -s ours FETCH_HEAD
git checkout -b update-to-g4-10.7.3 --no-track origin/master
git merge upstream-v10.7.3
# fix conflicts
git commit
git push -u origin update-to-g4-10.7.3

Note that I had to first make a dummy merge of v10.6.2 (our baseline) into v10.7.3 because the 10.7 and 10.6 branches diverged causing many fake conflicts.