replace DUNE_UNUSED by [[maybe_unused]]

Merged Bernd Flemisch requested to merge fix/dune-unused into releases/3.3

DUNE_UNUSED has been deprecated in Dune 2.8. As the header is not included in the Dune core modules anymore and we didn't include it explicitly in Dumux, this yields errors due to missing inclusions. Instead of including the deprecated header, better use [[maybe_unused]] instead.

Merge request reports