Fix Javadoc generation with Java 11

Author: nictas

pom.xml

@@ -148,6 +148,14 @@
                     <skip>false</skip>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <version>3.2.0</version>
+                <configuration>
+                    <source>8</source>
+                </configuration>
+            </plugin>
         </plugins>
     </build>