Bump mkdocs-material from 8.1.8 to 8.1.9

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 8.1.8 to 8.1.9.
- [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/8.1.8...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>

docs/requirements.txt

@@ -4,7 +4,7 @@ markdown==3.3.6
 mkdocs-awesome-pages-plugin==2.6.1
 mkdocs-git-revision-date-localized-plugin==0.11.1
 mkdocs-material-extensions==1.0.3
-mkdocs-material==8.1.8
+mkdocs-material==8.1.9
 mkdocs-minify-plugin==0.5.0
 mkdocs-redirects==1.0.3
 mkdocs==1.2.3