Bump mkdocs-material from 8.1.6 to 8.1.9

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 8.1.6 to 8.1.9.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@8.1.6...8.1.9)

---
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==1.2.3
-mkdocs-material==8.1.6
+mkdocs-material==8.1.9