Force classpath mode in Javadoc plugin.

maven-javadoc-plugin enables modulepath when running on Java 9+. That causes issues with JAR files that ship split packages. We now override the Java version property to pretend we must remain on classpath mode to prevent that dependencies with split-packages break the Javadoc build.

See #1537

Author: mp911de

parent/pom.xml

@@ -1272,6 +1272,7 @@
 					<header>${project.name}</header>
 					<source>${source.level}</source>
 					<quiet>true</quiet>
+					<javadocVersion>1.8</javadocVersion>
 					<!-- copies doc-files subdirectory which contains image resources -->
 					<docfilessubdirs>true</docfilessubdirs>
 					<doclint>html,reference</doclint>