Update to Asciidoc 4.0.0-alpha.1 (#855)

Author: puneetbehl

build.gradle

@@ -1,6 +1,6 @@
 plugins {
     id 'base'
-    id 'org.asciidoctor.convert' version '1.5.3'
+    id 'org.asciidoctor.jvm.convert' version '4.0.0-alpha.1'
     id 'org.grails.grails-gdoc-to-asciidoc'
 }
 
@@ -24,10 +24,6 @@ configurations {
     publish
 }
 
-asciidoctorj {
-    version = '1.5.4'
-}
-
 gdoc2asciidoc {
     apiLinks = ['org.hibernate':'https://docs.jboss.org/hibernate/orm/current/javadocs/',
                 'org.springframework.boot':'https://docs.spring.io/spring-boot/docs/2.5.2/api',