[Dune] dune-grid-howto is now archived

[Christoph Grüninger]

Dear fellow Dune users and developers!

With the Dune 2.7 release we deprecated dune-grid-howto, there was no 
2.8 release of this module.
I marked its repository now as archived. It is still there and can be 
cloned, but you cannot commit, the issues are frozen, and it does no 
longer show in the list of active core repositories.

Kind regards,
Christoph

-- 
L'enjeu est de bâtir la France de nos enfants,
pas de ressasser la France de notre enfance.
                        [Emmanuel Macron, 2022]