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@8.3.6...8.3.8)

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

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

Author: dependabot[bot]

gh-pages/requirements-dev.txt

@@ -1,4 +1,4 @@
 mkdocs~=1.3.0
 mkdocs-awesome-pages-plugin~=2.7.0
-mkdocs-material~=8.3.6
+mkdocs-material~=8.3.8
 mkdocs-git-revision-date-plugin~=0.3.2