org-ref：Version 3 has eliminated all the org-ref-* variables

[iT-Boyer]

org-ref：Version 3 has eliminated all the org-ref-* variables, and instead uses analogous variables defined in bibtex-completion.
org-ref-default-bibliography update to bibtex-completion-bibliography:

(use-package! org-ref
  :after-call calibredb
  :config
  (setq calibredb-ref-default-bibliography (concat (file-name-as-directory calibredb-root-dir) "catalog.bib"))
  (add-to-list 'bibtex-completion-bibliography calibredb-ref-default-bibliography)
  (setq org-ref-get-pdf-filename-function 'org-ref-get-mendeley-filename))