The jetty package to use. At the time of writing (v1.2023.12), PlantUML Server does not support Jetty versions higher than 12.x.
Jetty 12.x has introduced major breaking changes, see https://github.com/jetty/jetty.project/releases/tag/jetty-12.0.0 and https://eclipse.dev/jetty/documentation/jetty-12/programming-guide/index.html#pg-migration-11-to-12