Update asciidoctor-pdf requirement from 1.5.4 to 1.6.0

Updates the requirements on [asciidoctor-pdf](https://github.com/asciidoctor/asciidoctor-pdf) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-pdf/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-pdf@v1.5.4...v1.6.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.5.1'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-epub3', '1.5.0.alpha.19'
-gem 'asciidoctor-pdf', '1.5.4'
+gem 'asciidoctor-pdf', '1.6.0'
 
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.2.0'