Ref Guide PDF build throws Java warnings after requiring Java 11 on master

Since the master branch requires Java 11, the Ref Guide build has started throwing WARNINGs that we should look into. Despite the WARNINGs, the PDF build finishes successfully and appears the same as usual.

There are a couple moving parts here - first some classes hossman wrote to validate links in the PDF, and second a .jar from the asciidoctor-ant project. I suspect some of these are related to Hoss' code and some are from the project that's doing the PDF conversion.

I'll put the output from a Jenkins job in the comment to show the errors.