Rename `reference_datasets` to `reference_source_id`

## The problem

The ENSO Processes diagnostic has duplicate metric values appearing in the output (https://staging.climate-ref.org/diagnostics/pmp/enso_proc/scalars?detect_outliers=iqr&include_unverified=true).

Inspecting the output there is a `reference_datasets` facet which isn't in the CV and being ignored. We should change that to `reference_source_id` which is used by other diagnostics.
