Update mkdocs-minify-plugin requirement from <0.6 to <0.9

Updates the requirements on [mkdocs-minify-plugin](https://github.com/byrnereese/mkdocs-minify-plugin) to permit the latest version.
- [Release notes](https://github.com/byrnereese/mkdocs-minify-plugin/releases)
- [Commits](byrnereese/mkdocs-minify-plugin@0.1.0...0.8.0)

---
updated-dependencies:
- dependency-name: mkdocs-minify-plugin
  dependency-type: direct:production
...

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

requirements.txt

@@ -17,6 +17,6 @@ mkdocs-exclude>=1.0,<1.1
 mkdocs-git-authors-plugin<0.7
 mkdocs-git-revision-date-localized-plugin>=0.9,<1.1
 mkdocs-macros-plugin>=0.5,<0.7
-mkdocs-minify-plugin<0.6
+mkdocs-minify-plugin<0.9
 mkdocs-redirects<1.1
 mkdocs-rss-plugin>=0.20,<1.17