Bump asciidoctorj from 2.5.5 to 2.5.6 (#12705)

Bumps [asciidoctorj](https://github.com/asciidoctor/asciidoctorj) from 2.5.5 to 2.5.6.
- [Release notes](https://github.com/asciidoctor/asciidoctorj/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctorj/blob/v2.5.6/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctorj@v2.5.5...v2.5.6)

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

Signed-off-by: dependabot[bot] <[email protected]>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

grails-docs/build.gradle

@@ -12,7 +12,7 @@ dependencies {
             'org.yaml:snakeyaml:1.30',
             "org.codehaus.groovy:groovy-ant:$groovyVersion"
 
-    compile 'org.asciidoctor:asciidoctorj:2.5.5'
+    compile 'org.asciidoctor:asciidoctorj:2.5.6'
     compile('org.xhtmlrenderer:core-renderer:8.0') {
         exclude group: 'bouncycastle', module:'bcprov-jdk14'
     }