Replace NanoHTTPD with another lightweight HTTP server

[gaurav]

NanoHTTPD (https://github.com/NanoHttpd/nanohttpd) has not been updated in many years and has known vulnerabilities in Maven. We should replace it with a lightweight HTTP server that is being maintained, maybe Javalin. Note that we don't use the webserver command in the Klados backend, so AFAIK nobody actually uses JPhyloRef in this way, so this is not currently an urgent issue.