Improve PlantUML server config

What does this MR do and why?

Change Rewrite to reflect that the plantuml-server (especially the docker one) expects the image request on the root-path, e.g.:

http://plantuml:8080/png/XXXX

The old rewrite would have produced http://plantuml:8080/plantuml/png/XXXX, leading to an error

Extracted from community MR Fix plantuml-server rewrite (!113349 - closed) by @sigmund.dominik

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