Updated main version numbers to 2.0

src/idl_gen_java.cpp

@@ -591,7 +591,7 @@ class JavaGenerator : public BaseGenerator {
       // Force compile time error if not using the same version runtime.
       code += "  public static void ValidateVersion() {";
       code += " Constants.";
-      code += "FLATBUFFERS_1_12_0(); ";
+      code += "FLATBUFFERS_2_0_0(); ";
       code += "}\n";
 
       // Generate a special accessor for the table that when used as the root