Remove deprecations for 3.8

There are several deprecations:

dumux/discretization/cellcentered/mpfa/subcontrolvolumeface.hh:197:    [[deprecated("Will be removed after 3.8. Use fvGeometry.geometry(scvf).corners().")]]
dumux/discretization/cellcentered/mpfa/subcontrolvolumeface.hh:202:    [[deprecated("Will be removed after 3.8. Use fvGeometry.vertexCorner(scvf)")]]
dumux/discretization/cellcentered/mpfa/subcontrolvolumeface.hh:207:    [[deprecated("Will be removed after 3.8. Use fvGeometry.facetCorner(scvf)")]]
dumux/porousmediumflow/boxdfm/subcontrolvolume.hh:193:    [[deprecated("Will be removed after 3.7. Use fvGeometry.geometry(scv).")]]
dumux/porousmediumflow/boxdfm/subcontrolvolumeface.hh:228:    [[deprecated("Will be removed after 3.7. Use fvGeometry.geometry(scvf).")]]
dumux/linear/seqsolverbackend.hh:56:    [[deprecated("Removed after 3.8. Use solver from istlsolvers.hh")]]
dumux/linear/seqsolverbackend.hh:77:    [[deprecated("Removed after 3.8. Use solver from istlsolvers.hh")]]
dumux/linear/seqsolverbackend.hh:101:    [[deprecated("Removed after 3.8. Use solver from istlsolvers.hh")]]
dumux/linear/seqsolverbackend.hh:122:    [[deprecated("Removed after 3.8. Use solver from istlsolvers.hh")]]
dumux/multidomain/facet/box/subcontrolvolumeface.hh:179:    [[deprecated("Will be removed after 3.7. Use fvGeometry.geometry(scvf).")]]

Some are still from 3.7, probably the removal of them is not as straightforward.