Fix error on 'mvn_install_doc xx' for jdk {11, 12, 13}

[simonrommel]

Fixes the following error on mvn_install_doc xx when using jdk {11, 12, 13}

MavenReportException: Error while generating Javadoc:
Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in http://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.

As per https://bugs.openjdk.java.net/browse/JDK-8212233 this appears to be an issue when targeting java 8 and is fixed by specifying the source version in the configuration of the maven-javadoc-plugin.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-javadoc-plugin</artifactId>
    <configuration>
        <source>8</source>
    </configuration>
</plugin>