Add bytecode mapping for Java 13 through 16

15 is in early access.  16 will be soon enough.  Since this looks minimum for the error condition encountered now with 13 and above, this seems appropriate.

src/main/java/org/apache/maven/plugins/enforcer/EnforceBytecodeVersion.java

@@ -95,6 +95,22 @@ public class EnforceBytecodeVersion
         // Java 12
         JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "12", 56 );
         JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "1.12", 56 );
+
+        // Java 13
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "13", 57 );
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "1.13", 57 );
+
+        // Java 14
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "14", 58 );
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "1.14", 58 );
+
+        // Java 15
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "15", 59 );
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "1.15", 59 );
+
+        // Java 16
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "16", 60 );
+        JDK_TO_MAJOR_VERSION_NUMBER_MAPPING.put( "1.16", 60 );
     }
 
     static String renderVersion( int major, int minor )
@@ -218,7 +234,7 @@ private void computeParameters()
             if ( needle == null )
             {
                 throw new IllegalArgumentException( "Unknown JDK version given. Should be something like " +
-                        "\"1.7\", \"8\", \"11\", \"12\"" );
+                        "\"1.7\", \"8\", \"11\", \"12\", \"13\", \"14\", \"15\", \"16\"" );
             }
             maxJavaMajorVersionNumber = needle;
             if ( needle < 53 )