Previously when generating JATS with the `element_citations`
extension enabled, the references were put in a doubly-nested
ref-list element (`<ref-list><ref-list>...`). This is now fixed.
Closes#7990.
Citeproc adopted the BibTeX convention to use the author name "others"
when there are additional authors that are not named. JATS uses the
`<etal>` element for this.