Replace calls to deprecated `DUNE_VERSION_NEWER_REV(DUNE_ISTL,2,7,1)`
by `DUNE_VERSION_GT_REV(DUNE_ISTL,2,7,0)` which also reads better than
the `GTE` version, as Dune 2.7.1 hasn't been released yet. Fix the
call to `DUNE_VERSION_GT(DUNE_ISTL,2,7)` in the solver factory
backend which would break things in case of a 2.7.1 release. Adapt
the corresponding changelog entry.

Unfortunately the CMake logic for quad math detection was overwriting the Quadmath
package from Dune and had to be removed. This removes all quadmath code from
Dumux and uses the quadmath implementation of Dune instead (Dune::Float128)
which is based on the same library.

This reverts commit 37c21581.