build(deps): bump mkdocs-material in /hack/mkdocs/image (#4110)

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 9.6.18 to 9.6.20.
- [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...9.6.20)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-version: 9.6.20
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

hack/mkdocs/image/requirements.txt

@@ -10,7 +10,7 @@ MarkupSafe==3.0.2
 mkdocs==1.6.1
 mkdocs-awesome-pages-plugin==2.10.1
 mkdocs-macros-plugin==1.3.7
-mkdocs-material==9.6.18
+mkdocs-material==9.6.20
 mkdocs-redirects==1.2.2
 mkdocs-mermaid2-plugin==1.2.2
 pandas>=2.0.3