Update dependency org.asciidoctor:asciidoctorj-pdf to v1.6.0

This MR contains the following updates:

Package	Update	Change
org.asciidoctor:asciidoctorj-pdf	minor	1.5.4 -> 1.6.0

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Release Notes

asciidoctor/asciidoctorj-pdf

v1.6.0

asciidoctorj-pdf 1.6.0 brings the asciidoctor-pdf 1.6.0 gem to AsciidoctorJ.

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