Bump asciidoctorj-pdf from 2.1.4 to 2.1.6

Bumps [asciidoctorj-pdf](https://github.com/asciidoctor/asciidoctorj-pdf) from 2.1.4 to 2.1.6.
- [Release notes](https://github.com/asciidoctor/asciidoctorj-pdf/releases)
- [Commits](asciidoctor/asciidoctorj-pdf@v2.1.4...v2.1.6)

---
updated-dependencies:
- dependency-name: org.asciidoctor:asciidoctorj-pdf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pom.xml

@@ -83,7 +83,7 @@
     <maven.jar.plugin.version>3.2.2</maven.jar.plugin.version>
     <asciidoctor.maven.plugin.version>2.2.2</asciidoctor.maven.plugin.version>
     <asciidoctorj.version>2.5.4</asciidoctorj.version>
-    <asciidoctorj.pdf.version>2.1.4</asciidoctorj.pdf.version>
+    <asciidoctorj.pdf.version>2.1.6</asciidoctorj.pdf.version>
     <asciidoctorj.diagram.version>2.2.3</asciidoctorj.diagram.version>
     <buildnumber.plugin.version>1.4</buildnumber.plugin.version>
     <jruby.version>9.3.4.0</jruby.version>