Upgrade to asciidoctorj-pdf 1.6.0

[robertpanzer]

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Kind of change

• [ ] Bug fix
• [x] New non-breaking feature
• [ ] New breaking feature
• [ ] Documentation update
• [ ] Build improvement

Description

What is the goal of this pull request?

Upgrade asciidoctorj-pdf to 1.6.0.

How does it achieve that?

Are there any alternative ways to implement this?

Are there any implications of this pull request? Anything a user must know?

Issue

If this PR fixes an open issue, please add a line of the form:

Fixes #Issue

Release notes

Please add a corresponding entry to the file CHANGELOG.adoc