Bump org.asciidoctor:asciidoctorj-pdf from 2.3.20 to 2.3.21

Bumps [org.asciidoctor:asciidoctorj-pdf](https://github.com/asciidoctor/asciidoctorj-pdf) from 2.3.20 to 2.3.21.
- [Release notes](https://github.com/asciidoctor/asciidoctorj-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctorj-pdf/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctorj-pdf@v2.3.20...v2.3.21)

---
updated-dependencies:
- dependency-name: org.asciidoctor:asciidoctorj-pdf
  dependency-version: 2.3.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

docs/pom.xml

@@ -16,7 +16,7 @@
 
     <properties>
         <version.asciidoctor-maven-plugin>3.2.0</version.asciidoctor-maven-plugin>
-        <version.org.asciidoctor.asciidoctorj-pdf>2.3.20</version.org.asciidoctor.asciidoctorj-pdf>
+        <version.org.asciidoctor.asciidoctorj-pdf>2.3.21</version.org.asciidoctor.asciidoctorj-pdf>
         <version.org.asciidoctor.asciidoctorj-epub3>2.2.0</version.org.asciidoctor.asciidoctorj-epub3>
     </properties>