Fix maven-javadoc-plugin error: Specify the java version

And remove a redundant plugin

Author: nymanjens

pom.xml

@@ -161,10 +161,6 @@
 
   <build>
     <plugins>
-      <plugin>
-        <artifactId>maven-javadoc-plugin</artifactId>
-        <version>3.2.0</version>
-      </plugin>
       <plugin>
         <artifactId>maven-jar-plugin</artifactId>
         <version>3.2.0</version>
@@ -219,6 +215,9 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-javadoc-plugin</artifactId>
+            <configuration>
+              <source>8</source>
+            </configuration>
             <version>3.2.0</version>
             <executions>
               <execution>