Bump mkdocs-material from 9.5.18 to 9.5.21

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.5.18 to 9.5.21.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.5.18...9.5.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements-test.txt

@@ -4,7 +4,7 @@ isort==5.13.2
 bandit==1.7.8
 mike==2.0.0
 markdown-include==0.8.1
-mkdocs-material==9.5.18
+mkdocs-material==9.5.21
 mkdocs-minify-plugin==0.8.0
 mkdocstrings==0.24.3
 ./util/mkdocs-redirects