Bump mkdocs-material from 9.1.19 to 9.1.21

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.1.19 to 9.1.21.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.1.19...9.1.21)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,2 +1,2 @@
 mkdocs==1.4.3
-mkdocs-material==9.1.19
+mkdocs-material==9.1.21