Bump mkdocs-material from 9.6.11 to 9.6.21

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.6.11 to 9.6.21.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.6.11...9.6.21)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-version: 9.6.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -13,7 +13,7 @@
     python_requires='>=3.11, <4',
     install_requires=[
         "mkdocs==1.6.1",
-        "mkdocs-material==9.6.11",
+        "mkdocs-material==9.6.21",
         "mkdocs-minify-plugin==0.8.0",
         "mkdocs-redirects==1.2.2",
     ],