Add JUL support to log4j

Certain badly behaved plugins insist on logging with java.util.logging. Accommodate them.

proxy/build.gradle

@@ -57,6 +57,7 @@ dependencies {
     implementation "org.apache.logging.log4j:log4j-core:${log4jVersion}"
     implementation "org.apache.logging.log4j:log4j-slf4j-impl:${log4jVersion}"
     implementation "org.apache.logging.log4j:log4j-iostreams:${log4jVersion}"
+    implementation "org.apache.logging.log4j:log4j-jul:${log4jVersion}"
 
     implementation 'net.sf.jopt-simple:jopt-simple:5.0.4' // command-line options
     implementation 'net.minecrell:terminalconsoleappender:1.2.0'

proxy/src/main/java/com/velocitypowered/proxy/Velocity.java

@@ -8,9 +8,12 @@ import org.apache.logging.log4j.Logger;
 
 public class Velocity {
 
-  private static final Logger logger = LogManager.getLogger(Velocity.class);
+  private static final Logger logger;
 
   static {
+    System.setProperty("java.util.logging.manager", "org.apache.logging.log4j.jul.LogManager");
+    logger = LogManager.getLogger(Velocity.class);
+
     // We use BufferedImage for favicons, and on macOS this puts the Java application in the dock.
     // How inconvenient. Force AWT to work with its head chopped off.
     System.setProperty("java.awt.headless", "true");