Bump org.asciidoctor:asciidoctorj-pdf from 2.3.20 to 2.3.21 (#627)

Co-authored-by: Boni Garcia <boni.garcia@uc3m.es>

Author: dependabot[bot]

pom.xml

@@ -30,7 +30,7 @@
  <maven-gpg-plugin.version>3.2.8</maven-gpg-plugin.version>
  <exec-maven-plugin.version>3.0.0</exec-maven-plugin.version>
  <maven-asciidoctor-plugin.version>3.2.0</maven-asciidoctor-plugin.version>
- <asciidoctorj-pdf.version>2.3.20</asciidoctorj-pdf.version>
+ <asciidoctorj-pdf.version>2.3.21</asciidoctorj-pdf.version>
  <maven-site.version>3.21.0</maven-site.version>
  <maven-project-info-reports.version>3.9.0</maven-project-info-reports.version>
  <maven-javadoc-plugin.version>3.12.0</maven-javadoc-plugin.version>