Update maven-javadoc-plugin to 3.0.0.
Update maven-javadoc-plugin to 3.0.0.

This is necessary to build with JDK9.
Or rather, it *would* be necessary if not for your override of the plexus-io dependency. But the override feels like something that would break eventually, so it seems worth simplifying this while I'm updating our other projects to 3.0.0.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182817606

google/error-prone,cushon/error-prone,cushon/error-prone,cushon/error-prone,cushon/error-prone,google/error-prone