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] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -46,7 +46,7 @@ dev-mkdocs = [
   "mike == 1.1.2",
   "mkdocs-gen-files == 0.5.0",
   "mkdocs-literate-nav == 0.6.0",
-  "mkdocs-material == 9.1.19",
+  "mkdocs-material == 9.1.21",
   "mkdocs-section-index == 0.3.5",
   "mkdocstrings[python] == 0.22.0",
   "frequenz-repo-config[lib] == 0.3.0",