[Opm] Dropping DUNE 2.6 support for OPM 2022.10?

[Atgeirr Rasmussen]

Hi,

Circling back to Arne Morten's point, that we would have to backport Dune and put it in our own ppa to use the newer Dune releases, how much work would that be? And if we did that, could/should we skip straight to Dune 2.8 or even 2.9? If we set as a goal to:
 - Use and require as new Dune as feasible (say 2.8 for now).
 - Use and require as recent C++ standard as feasible (we now require C++17, but say we want to use C++20).
Other than developers updating their systems (which I consider quite acceptable as long as we do not require bleeding-edge versions), and the above-mentioned backporting, what else would we need to do? I assume Jenkins and Debian may be points of difficulty.

Atgeirr