Changes

#1191: Do not start on pre-release Java 17

Pre-release versions of Java may miss standard API that is introduced later during development of the same release. Unfortunately some server hosts knowingly (!) run these versions in production, even years after the general availability of that java release. Therefore, we need to manually enforce that the server only runs on fully standardized versions to ensure compatibility of plugin and server code with the runtime.

To cause less problems when developers test their software with newer Java, this rule is only enforced on the oldest Java release the server can run with. This is the Java version all plugins will normally compile to, so it's feature set must always be available during runtime.