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.
Please backport to 3.2 @nedc.