Aquire all system labels from Semantic Background

[nicholascar]

All the vocabs and ontologies in [context_ontologies](https://github.com/RDFLib/prez/tree/main/prez/reference_data/ context_ontologies) and all of the prefixes in prefixes that are generic, i.e. useful beyond Prez, should come from https://github.com/Kurrawong/semantic-background. This way we can maintain one set of labels only.