Update mkdocs-material to v9

The currently used version of mkdocs-material is from September 2022. It would be great to update to a more recent version. However, please mind that this requires changes to the projects' mkdocs.yml, i.e. it will require some work from users: https://squidfunk.github.io/mkdocs-material/upgrade/#upgrading-from-8x-to-9x