CompatHelper: bump compat for Oceananigans to 0.91, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Oceananigans package from 0.90.10 to 0.90.10, 0.91. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.