[Opm] Release 2023.04

[Arne Morten Kvarving]

 Dear OPM community,

  It is time to create the next OPM release, we intend to follow this schedule, all days in April:

  Monday 10: Final merges to master before branching, freeze the master branch.
  Wednesday 12: Create release branches, unfreeze the master branch.
  Monday 17: First release candidate ready for testing.

  The release will then follow shortly after testing, if all is well. If not, we may have further release candidates.

 All developers, please mark all PRs you would like to see merged before the release with the "Release 2023.04" milestone.

Arne Morten Kvarving