Bump mkdocs-material from 9.6.18 to 9.6.21

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.6.18 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@9.6.18...9.6.21)

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

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Author: dependabot[bot]