See #3145.

Main steps (may be incomplete, needs decision on timeline):

• Deprecate DataArray.attrs see Deprecate DataArray.attrs #3227
• Deprecation period? -> 24.12.0
• Update Scipp docs
• How can we avoid backsliding?
• Deprecate scippneutron.load, add scippneutron.load_with_mantid loading as DataGroup see Change load_with_mantid and from_mantid to return data groups scippneutron#444
• Change plopp to only look at coords, not attrs. see Remove attrs use plopp#241
• Removing attrs before we have a workflow system implemented would mean duplicate refactoring effort for certain data-reduction workflows
• Refactor scippuncertainties
• Document scope of Scipp.
• Consider how to handle loading of legacy Scipp HDF5 files. Should we load old files that have attrs as DataGroup? -> mainly used internally, ignore the problem

Alignment-flag refactor related:

• Change scippnexus slicing behavior.