Update version of mermaid

Obviously, different versions of mkdocs require different mermaid versions (See also https://github.com/squidfunk/mkdocs-material/blob/master/src/templates/assets/javascripts/components/content/mermaid/index.ts#L72).

In case mermaid does not work, the old mermaid.min.js from directory doc.zih.tu-dresden.de/tud_theme/javascripts needs to be removed, so that the new version is downloaded. (By default, the download script only downloads mermaid.min.js again, if it is not there.)

Closes #594 (closed)