fix(deps): update mkdocs-material to 9.1.21

| datasource | package         | from   | to     |
| ---------- | --------------- | ------ | ------ |
| pypi       | mkdocs-material | 9.1.20 | 9.1.21 |

.ci/mkdocs/requirements.txt

@@ -1,6 +1,6 @@
 mkdocs==1.5.0
 mkdocs-macros-plugin==1.0.2
-mkdocs-material==9.1.20
+mkdocs-material==9.1.21
 mkdocs-minify-plugin==0.7.0
 mkdocs-redirects==1.2.1
 mkdocs-awesome-pages-plugin==2.9.1