chore(deps-dev): Update mkdocs-material requirement in /gh-pages

Updates the requirements on [mkdocs-material](https://github.com/squidfunk/mkdocs-material) to permit the latest version.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](squidfunk/mkdocs-material@9.1.19...9.1.21)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-type: direct:development
...

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

gh-pages/requirements-dev.txt

@@ -1,4 +1,4 @@
 mkdocs~=1.4.3
 mkdocs-awesome-pages-plugin~=2.9.1
-mkdocs-material~=9.1.19
+mkdocs-material~=9.1.21
 mkdocs-git-revision-date-plugin~=0.3.2