Fix dependencies for PluginServlet on Java 11
Description
When running the Rest API Plugin, any request is met with an HTTP 500, and the following is logged to the console:
There are bugs logged in the RestAPI repo reporting this:
https://github.com/igniterealtime/openfire-restAPI-plugin/issues/25
https://github.com/igniterealtime/openfire-restAPI-plugin/issues/39
https://github.com/igniterealtime/openfire-restAPI-plugin/issues/46
Environment
Java 11
