[cleanup] Use quad-precision type support from upstream dune-common
Dune introduced a quad-precision type with 2.7 (which we now depend on): https://gitlab.dune-project.org/core/dune-common/-/blob/master/dune/common/quadmath.hh
This should be tested as replacement for https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/dumux/common/quad.hh