Bump Python requirement mkdocs-material to 9.1.21

docs-requirements.txt

 black==23.7.0
 doc8==1.1.1
 mkdocs==1.5.2
-mkdocs-material==9.1.14
+mkdocs-material==9.1.21
 mkdocstrings==0.21.2
 mkdocstrings-python==1.3.0
 recommonmark==0.7.1