dune/fufem/assemblers/istlbackend.hh: shiftedMultiIndex gone from dune-functions

Building dune-fufem fails as shiftedMultiIndex was removed from dune-functions:

.../dune-fufem/dune/fufem/assemblers/istlbackend.hh:179:28: error: 'shiftedMultiIndex' is not a member of 'Dune::Functions'
       auto&& subColIndex = Dune::Functions::shiftedMultiIndex(colIndex_);

(from https://git.imp.fu-berlin.de/burchardt_at_igpm.rwth-aachen.de/dune-fufem/-/jobs/9376)