Upgrade to Asciidoctor Gradle Plugin 2.4

This commit updates the build to use the latest version of the
Asciidoctor Gradle Plugin. One significant new feature is that the
plugin's tasks are now cacheable.

Closes gh-24216

build.gradle

@@ -1,15 +1,8 @@
-buildscript {
-	dependencies {
-		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16'
-		classpath 'io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.3.RELEASE'
-	}
-}
-
 plugins {
 	id 'io.spring.dependency-management' version '1.0.8.RELEASE' apply false
 	id 'org.jetbrains.kotlin.jvm' version '1.3.61' apply false
 	id 'org.jetbrains.dokka' version '0.9.18' apply false
-	id 'org.asciidoctor.convert' version '1.5.8'
+	id 'org.asciidoctor.jvm.convert' version '2.4.0'
 	id 'io.spring.nohttp' version '0.0.4.RELEASE'
 	id 'de.undercouch.download' version '4.0.0'
 	id 'com.gradle.build-scan' version '3.1.1'
@@ -413,10 +406,6 @@ configure(rootProject) {
 		}
 	}
 
-	dependencies {
-		asciidoctor("io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.3.RELEASE")
-	}
-
 	publishing {
 		publications {
 			mavenJava(MavenPublication) {

gradle/docs.gradle

@@ -1,3 +1,11 @@
+configurations {
+	asciidoctorExt
+}
+
+dependencies {
+	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions:0.2.0.RELEASE")
+}
+
 /**
  * Produce Javadoc for all Spring Framework modules in "build/docs/javadoc"
  */
@@ -93,11 +101,21 @@ task extractDocResources(type: Copy, dependsOn: downloadResources) {
 	into "$buildDir/docs/spring-docs-resources/"
 }
 
+asciidoctorj {
+	modules {
+		pdf {
+			version '1.5.0-beta.8'
+		}
+	}
+}
+
 /**
  * Produce the Spring Framework Reference documentation
  * from "src/docs/asciidoc" into  "build/asciidoc/html5"
  */
 asciidoctor {
+	baseDirFollowsSourceDir()
+	configurations 'asciidoctorExt'
 	sources {
 		include '*.adoc'
 	}
@@ -106,13 +124,15 @@ asciidoctor {
 		from(sourceDir) {
 			include 'images/*', 'css/**', 'js/**'
 		}
-		from "$buildDir/docs/spring-docs-resources/"
+		from extractDocResources
 	}
 	logDocuments = true
-	backends = ["html5"]
-	// only ouput PDF documentation for non-SNAPSHOT builds
-	if (!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
-		backends += "pdf"
+	outputOptions {
+		backends = ["html5"]
+		// only ouput PDF documentation for non-SNAPSHOT builds
+		if (!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
+			backends += "pdf"
+		}
 	}
 	options doctype: 'book', eruby: 'erubis'
 	attributes([
@@ -132,8 +152,6 @@ asciidoctor {
 	])
 }
 
-asciidoctor.dependsOn extractDocResources
-
 /**
  * Zip all docs (API and reference) into a single archive
  */