Don't use javadoc -html5 arg yet
Don't use javadoc -html5 arg yet

The javadoc -html5 argument requires JDK 9 or JDK 10.

Breaks compile on Java 8, though it did not break compile on
ci.jenkins.io.

MarkEWaite/git-client-plugin,MarkEWaite/git-client-plugin,MarkEWaite/git-client-plugin