use standard `[[deprecated(...)]]` attribute

merged

mentioned in commit d3a70626

For the sake of completeness: In dune-common we still use the Dune macro because GCC 4.9 had some issues with the standrd deprecation attribute. At least Steffen told me this a while ago.

@gruenich: DUNE master (2.7) no longer supports GCC 4.9, see core/dune-common!320 (merged) and core/dune-common#91 (closed)