Release 2.6

This issue is for general pre release discussion. Please work through the list of all issues and MRs tagged with the Release 2.6 Milestone

https://gitlab.dune-project.org/dashboard/milestones/dune-260?title=DUNE+2.6.0#tab-issues

If something is missing from that list, then please tag it, but also remove tags from issues/MRs that will not make it any more into the 2.6 release.