Downgrade mkdocs-bibtex for faster builds

requirements-docs.txt

@@ -7,7 +7,7 @@ mkdocstrings[python]==0.27
 mkdocstrings-python==1.13.0
 mkdocs-alias-plugin==0.8.1
 mkdocs-autorefs==1.3.0
-mkdocs-bibtex==4.1.0
+mkdocs-bibtex==2.21.0
 mkdocs-gen-files==0.5.0
 mkdocs-git-revision-date-localized-plugin
 mkdocs-glightbox==0.4.0