Upgrade to AsciidoctorJ 3.1.0

Closes gh-24991

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."