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](https://github.com/asciidoctor/asciidoctorj-pdf/compare/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>

spec/pom.xml

diff --git a/spec/pom.xml b/spec/pom.xml
index 9f42812..6f7a06f 100644
--- a/spec/pom.xml
+++ b/spec/pom.xml
@@ -89,7 +89,7 @@
         <site.output.dir>${project.build.directory}/staging</site.output.dir>
         <maven.site.skip>true</maven.site.skip>
         <asciidoctorj.version>3.0.0</asciidoctorj.version>
-        <asciidoctorj.pdf.version>2.3.20</asciidoctorj.pdf.version>
+        <asciidoctorj.pdf.version>2.3.21</asciidoctorj.pdf.version>
         <jruby.version>10.0.2.0</jruby.version>
         <!-- status: DRAFT, BETA, etc., or blank for final -->
         <status>DRAFT</status>