Remove desktop file, as it's unnecessary for a primarily terminal-based program. Upgrade to the latest version that fully supports being built with JDK 8.
This is the newest version I could upgrade to without build errors due to various factors. Gradle versions past 4.9 either fail to compile without JDK 9+, or outright refuse to compile without JDK 9+. This Gradle update was necessary to upgrade OpenJFX 8.