chore(deps): bump mkdocs-material in the mkdocs group

Bumps the mkdocs group with 1 update: [mkdocs-material](https://github.com/squidfunk/mkdocs-material).


Updates `mkdocs-material` from 9.6.20 to 9.6.21
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Commits](https://github.com/squidfunk/mkdocs-material/compare/9.6.20...9.6.21)

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

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

docs/requirements-docs.txt

@@ -1,6 +1,6 @@
 markdown==3.9
 mkdocs==1.6.1
-mkdocs-material==9.6.20
+mkdocs-material==9.6.21
 mdx_truly_sane_lists==1.3
 pymdown-extensions==10.16.1
 jinja2==3.1.6