Effectively convert Javadoc 8 lint errors into warnings

When we upgraded to Java 8 we also set a Maven property (javadoc.additional.params) to tell the Javadoc linter to be less strict about some problems.

However, later we upgraded to base-pom >= 4.0, which removes the support for this Maven property (part of the changes made in https://extranet.atlassian.com/jira/browse/BUILDENG-8535). This is causing the Javadoc build to fail.