Bump Python requirement mkdocstrings to 0.21.2

docs-requirements.txt

@@ -2,6 +2,6 @@ black==23.3.0
 doc8==1.1.1
 mkdocs==1.4.2
 mkdocs-material==9.1.6
-mkdocstrings==0.20.0
+mkdocstrings==0.21.2
 mkdocstrings-python==0.9.0
 recommonmark==0.7.1