Upgrade Kroki server

The diagrams on the CBO page for Query are marked up using PlantUML. Our own kroki server cannot render them, and returns 404 for each image. The public kroki server renders them without any problem.

I reckon these diagrams contain newer markup which our old kroki server cannot deal with. Unfortunately the PlantUML documentation is versionless, so I have no way of knowing which markup is supported by our kroki server without trial and error.