Replace NashornScriptEngine with implicit GraalJSScriptEngine

GraalJSScriptEngine implements Invocable, which is why casting to it works.
However, this is just a quick and ugly fix to make it compile.
A better solution would be to cast it once, immediately after `eval`,
and from then on only handle it as Invocable.

Scripts still need to be fixed. They are still using Rhino and Nashorn-specific
ways of importing packages.
Usages of "importPackage" and "Packages" need to be replaced with
the Graal specific "Java.type".

src/main/java/constants/net/ServerConstants.java

@@ -5,32 +5,6 @@ public class ServerConstants {
     //Server Version
     public static short VERSION = 83;
 
-    //Java Configuration
-    public static final boolean JAVA_8 = getJavaVersion() >= 8;         //Max amount of times a party leader is allowed to persist on the Party Search before entry expiration (thus needing to manually restart the Party Search to be able to search for members).
-    
     //Debug Variables
     public static int[] DEBUG_VALUES = new int[10];             // Field designed for packet testing purposes
-    
-    // https://github.com/openstreetmap/josm/blob/a3a6e8a6b657cf4c5b4c64ea14d6e87be6280d65/src/org/openstreetmap/josm/tools/Utils.java#L1566-L1585
-    // Added by kolakcc (Familiar)
-    /**
-     * Returns the Java version as an int value.
-     * @return the Java version as an int value (8, 9, etc.)
-     * @since 12130
-     */
-    public static int getJavaVersion() {
-        String version = System.getProperty("java.version");
-        if (version.startsWith("1.")) {
-            version = version.substring(2);
-        }
-        // Allow these formats:
-        // 1.8.0_72-ea
-        // 9-ea
-        // 9
-        // 9.0.1
-        int dotPos = version.indexOf('.');
-        int dashPos = version.indexOf('-');
-        return Integer.parseInt(version.substring(0,
-                dotPos > -1 ? dotPos : dashPos > -1 ? dashPos : 1));
-    }
 }