Bump mkdocs-material from 9.1.20 to 9.1.21 (#629)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.1.20 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@9.1.20...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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,5 +1,5 @@
 pymdown-extensions==10.1
 pygments==2.15.1
-mkdocs-material==9.1.20
+mkdocs-material==9.1.21
 mkdocs==1.5.0
 mkdocs-cluster==0.0.9