Update to Zola >0.22

We are currently on zola 0.20 while 0.22.1 is current.

- We need to coordinate with SREs to apply the update inconjunction with a config update:
- An update from https://github.com/getzola/giallo removed the visual-studio-dark theme we are using for our syntax highlighting, making this update non-trivial. We have to pick a replacement and make sure we don't have anything else depending on it (e.g. matching colors set manually).
- We also need to look into "Only outputting CSS classes like before 0.22 is not possible anymore."

Start by locating the Zola version and site configuration, then inspect references to the removed visual-studio-dark theme and the CSS class output described in the issue. Coordinate with SREs on the configuration update; done means Zola 0.22.1 works with a replacement theme and no dependent color or class behavior is broken.

Written by the indexing model from the issue text.