Bump org.asciidoctor:asciidoctorj-pdf from 2.3.13 to 2.3.15

Bumps [org.asciidoctor:asciidoctorj-pdf](https://github.com/asciidoctor/asciidoctorj-pdf) from 2.3.13 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@v2.3.13...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>

build.gradle

@@ -15,7 +15,7 @@ repositories {
 
 ext {
   asciidoctorjVersion = '2.5.11'
-  asciidoctorjPdfVersion = '2.3.13'
+  asciidoctorjPdfVersion = '2.3.15'
 }
 
 dependencies {