Update to asciidoctor-gradle-plugin:1.5.7

Fixes gh-8264

build.gradle

@@ -9,7 +9,7 @@ buildscript {
 		classpath("io.spring.gradle:spring-io-plugin:0.0.8.RELEASE")
 		classpath("com.bmuschko:gradle-tomcat-plugin:2.2.4")
 		classpath('me.champeau.gradle:gradle-javadoc-hotfix-plugin:0.1')
-		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.5.1')
+		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.5.7')
 		classpath("io.spring.gradle:docbook-reference-plugin:0.3.1")
 		classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.19.RELEASE")
 		classpath "org.jfrog.buildinfo:build-info-extractor-gradle:4.12.0"