build(deps-dev): bump mkdocs-material from 9.1.19 to 9.1.21 (#376)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pyproject.toml

@@ -45,7 +45,7 @@ publish = [
 docs = [
     "black==23.7.0",
     "mkdocs==1.5.1",
-    "mkdocs-material==9.1.19",
+    "mkdocs-material==9.1.21",
     "mkdocstrings[python]==0.22.0",
     "mkdocstrings-python==1.2.1",
     "markdown-include==0.8.1",