Upgrade to AsciidoctorJ 2.5.7

Closes gh-872

Author: wilkinsona

spring-restdocs-platform/build.gradle

@@ -15,7 +15,7 @@ dependencies {
 		api("org.apache.pdfbox:pdfbox:2.0.27")
 		api("org.apache.tomcat.embed:tomcat-embed-core:10.1.1")
 		api("org.apache.tomcat.embed:tomcat-embed-el:10.1.1")
-		api("org.asciidoctor:asciidoctorj:2.5.6")
+		api("org.asciidoctor:asciidoctorj:2.5.7")
 		api("org.asciidoctor:asciidoctorj-pdf:2.3.0")
 		api("org.assertj:assertj-core:3.23.1")
 		api("org.hamcrest:hamcrest-core:1.3")