Bump mkdocs-material from 9.1.18 to 9.1.21 (#165)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.1.18 to 9.1.21.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](https://github.com/squidfunk/mkdocs-material/compare/9.1.18...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-docs.txt

@@ -1,5 +1,5 @@
 mkdocs==1.4.2
-mkdocs-material==9.1.18
+mkdocs-material==9.1.21
 pymdown-extensions==10.0
 mike==1.1.2
 mkdocs-simple-hooks==0.1.5