Bump org.asciidoctor:asciidoctorj-pdf from 2.3.7 to 2.3.9

Bumps [org.asciidoctor:asciidoctorj-pdf](https://github.com/asciidoctor/asciidoctorj-pdf) from 2.3.7 to 2.3.9.
- [Release notes](https://github.com/asciidoctor/asciidoctorj-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctorj-pdf/blob/main/CHANGELOG.adoc)
- [Commits](https://github.com/asciidoctor/asciidoctorj-pdf/commits/v2.3.9)

---
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>

build.gradle

@@ -15,7 +15,7 @@ buildscript {
 
     dependencies {
         classpath "org.asciidoctor:asciidoctor-gradle-jvm:$asciidoctorGradlePluginVersion"
-        classpath 'org.asciidoctor:asciidoctorj-pdf:2.3.7'
+        classpath 'org.asciidoctor:asciidoctorj-pdf:2.3.9'
         classpath "org.beryx:badass-jar:2.0.0"
         classpath 'biz.aQute.bnd:biz.aQute.bnd.gradle:6.4.0'
         classpath "io.codearte.gradle.nexus:gradle-nexus-staging-plugin:0.30.0"