Bump mkdocs-material from 9.6.14 to 9.6.21

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.6.14 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@9.6.14...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]

docs/requirements.txt

@@ -1,6 +1,6 @@
 mkdocs==1.6.1
 mkdocs-git-revision-date-localized-plugin==1.4.7
-mkdocs-material==9.6.14
+mkdocs-material==9.6.21
 mkdocs-material-extensions==1.3.1
 mkdocs-minify-plugin==0.8.0
 mkdocs-redirects==1.2.2