📦 deps: bump mkdocs-material from 9.6.18 to 9.6.21 (#86)

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
...

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

Author: dependabot[bot]

requirements.txt

@@ -2,4 +2,4 @@ clishelf==0.2.23
 mkdocs-git-committers-plugin-2==2.5.0
 mkdocs-git-revision-date-localized-plugin==1.4.7
 mkdocs-glightbox==0.4.0
-mkdocs-material==9.6.18
+mkdocs-material==9.6.21