Stale read on CreateDatasetElement

Refs https://redmine.teklia.com/issues/7160

A stale read on CreateDatasetElement prevented an arkindex ml-split CLI command to complete succesfully.

It probably comes from the lack of using("default") on the set retrieval in DatasetElementSerializer. As sets were previously stored directly on the dataset, it makes sense that we have a regression since 1.6.0