Remove support for Dune 2.6 and older

.gitlab-ci.yml

dune.module

dune/gfe/cosseratvtkwriter.hh

dune/gfe/geodesicfefunctionadaptor.hh

dune/gfe/linearalgebra.hh

dune/gfe/localprojectedfefunction.hh

dune/gfe/mixedgfeassembler.hh

dune/gfe/periodic1dpq1nodalbasis.hh

dune/gfe/riemannianpnsolver.cc

dune/gfe/riemannianpnsolver.hh

dune/gfe/riemanniantrsolver.cc

dune/gfe/riemanniantrsolver.hh

dune/gfe/unitvector.hh

dune/gfe/vtkfile.hh

src/cosserat-continuum.cc

src/film-on-substrate.cc

src/harmonicmaps.cc

test/CMakeLists.txt

test/harmonicmaptest.cc