Prepare for imminent Java 12 release

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/Main.java

@@ -148,8 +148,8 @@ public class Main {
             }
 
             float javaVersion = Float.parseFloat(System.getProperty("java.class.version"));
-            if (javaVersion > 55.0) {
-                System.err.println("Unsupported Java detected (" + javaVersion + "). Only up to Java 11 is supported.");
+            if (javaVersion > 56.0) {
+                System.err.println("Unsupported Java detected (" + javaVersion + "). Only up to Java 12 is supported.");
                 return;
             }