Validate other versions of CITATION.cff files

[fwkoch]

Currently this only validates 1.2.0 against the latest json schema. Older versions define yaml schema (e.g. https://github.com/citation-file-format/schema/blob/1.0.3/CFF-Core/schema.yaml); supporting these would require a separate yaml schema validation library. I don't know to what extent older versions are used vs. 1.2.0, so we should assess that before putting in too much work on this.