Make block switches work on every page of multi-page docs

There's a bug in some versions of AsciidoctorJ where extensions are
only called on this first conversion performed by a given Asciidoctor
instance. This affects the multi-page documentation where there are
multiple conversions performed – one for each page in the
documentation.

This commit upgrades the version of AsciidoctorJ used by the
Asciidoctor Gradle plugin to 2.4.1, which contains fix for the problem.

Closes gh-23709

Author: wilkinsona

buildSrc/src/main/java/org/springframework/boot/build/AsciidoctorConventions.java

@@ -46,6 +46,7 @@
  *
  * <ul>
  * <li>All warnings are made fatal.
+ * <li>The version of AsciidoctorJ is upgraded to 2.4.1.
  * <li>A task is created to resolve and unzip our documentation resources (CSS and
  * Javascript).
  * <li>For each {@link AsciidoctorTask} (HTML only):
@@ -69,10 +70,13 @@
  */
 class AsciidoctorConventions {
 
+	private static final String ASCIIDOCTORJ_VERSION = "2.4.1";
+
 	void apply(Project project) {
 		project.getPlugins().withType(AsciidoctorJPlugin.class, (asciidoctorPlugin) -> {
 			configureDocResourcesRepository(project);
 			makeAllWarningsFatal(project);
+			upgradeAsciidoctorJVersion(project);
 			UnzipDocumentationResources unzipResources = createUnzipDocumentationResourcesTask(project);
 			project.getTasks().withType(AbstractAsciidoctorTask.class, (asciidoctorTask) -> {
 				configureCommonAttributes(project, asciidoctorTask);
@@ -110,6 +114,10 @@ private void makeAllWarningsFatal(Project project) {
 		project.getExtensions().getByType(AsciidoctorJExtension.class).fatalWarnings(".*");
 	}
 
+	private void upgradeAsciidoctorJVersion(Project project) {
+		project.getExtensions().getByType(AsciidoctorJExtension.class).setVersion(ASCIIDOCTORJ_VERSION);
+	}
+
 	private UnzipDocumentationResources createUnzipDocumentationResourcesTask(Project project) {
 		Configuration documentationResources = project.getConfigurations().maybeCreate("documentationResources");
 		documentationResources.getDependencies()