Upgrade to AsciidoctorJ 2.5.6

Closes gh-857

Author: wilkinsona

spring-restdocs-platform/build.gradle

@@ -13,7 +13,7 @@ dependencies {
 		api("jakarta.validation:jakarta.validation-api:3.0.0")
 		api("junit:junit:4.12")
 		api("org.apache.pdfbox:pdfbox:2.0.27")
-		api("org.asciidoctor:asciidoctorj:2.5.4")
+		api("org.asciidoctor:asciidoctorj:2.5.6")
 		api("org.asciidoctor:asciidoctorj-pdf:2.1.4")
 		api("org.assertj:assertj-core:3.23.1")
 		api("org.hamcrest:hamcrest-core:1.3")