Upgrade to AsciidoctorJ 3.1.0

Closes spring-projectsgh-24991

Author: bclozel

build.gradle

@@ -3,8 +3,8 @@ plugins {
 	id 'io.spring.nohttp' version '0.0.5.RELEASE'
 	id 'org.jetbrains.kotlin.jvm' version '1.4.10' apply false
 	id 'org.jetbrains.dokka' version '0.10.1' apply false
-	id 'org.asciidoctor.jvm.convert' version '2.4.0'
-	id 'org.asciidoctor.jvm.pdf' version '2.4.0'
+	id 'org.asciidoctor.jvm.convert' version '3.1.0'
+	id 'org.asciidoctor.jvm.pdf' version '3.1.0'
 	id 'de.undercouch.download' version '4.1.1'
 	id "io.freefair.aspectj" version '5.1.1' apply false
 	id "com.github.ben-manes.versions" version '0.28.0'

gradle/docs.gradle

@@ -3,7 +3,7 @@ configurations {
 }
 
 dependencies {
-	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions-block-switch:0.4.2.RELEASE")
+	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions-block-switch:0.4.3.RELEASE")
 }
 
 repositories {
@@ -124,11 +124,7 @@ task extractDocResources(type: Copy, dependsOn: downloadResources) {
 }
 
 asciidoctorj {
-	modules {
-		pdf {
-			version '1.5.0-beta.8'
-		}
-	}
+	version = '2.4.1'
 	fatalWarnings ".*"
 	options doctype: 'book', eruby: 'erubis'
 	attributes([
@@ -153,7 +149,6 @@ asciidoctorj {
  * in "build/docs/ref-docs/html5".
  */
 asciidoctor {
-	dependsOn asciidoctorPdf
 	baseDirFollowsSourceDir()
 	configurations 'asciidoctorExt'
 	sources {
@@ -186,7 +181,7 @@ asciidoctorPdf {
 /**
  * Zip all docs (API and reference) into a single archive
  */
-task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'dokka']) {
+task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'asciidoctorPdf', 'dokka']) {
 	group = "Distribution"
 	description = "Builds -${archiveClassifier} archive containing api and reference " +
 			"for deployment at https://docs.spring.io/spring-framework/docs."