Bump org.asciidoctor:asciidoctorj-pdf from 2.3.9 to 2.3.15

Bumps [org.asciidoctor:asciidoctorj-pdf](https://github.com/asciidoctor/asciidoctorj-pdf) from 2.3.9 to 2.3.15.
- [Release notes](https://github.com/asciidoctor/asciidoctorj-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctorj-pdf/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctorj-pdf@v2.3.9...v2.3.15)

---
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>

spec/pom.xml

@@ -35,7 +35,7 @@
         <site.output.dir>${project.build.directory}/staging</site.output.dir>
         <maven.site.skip>true</maven.site.skip>
         <asciidoctorj.version>2.5.12</asciidoctorj.version>
-        <asciidoctorj.pdf.version>2.3.9</asciidoctorj.pdf.version>
+        <asciidoctorj.pdf.version>2.3.15</asciidoctorj.pdf.version>
         <jruby.version>9.3.13.0</jruby.version>
         <!-- status: DRAFT, BETA, etc., or blank for final -->
         <status>DRAFT</status>