Use highlightjs for syntax highlighting in Asciidoctor's HTML output

Closes gh-21701

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

@@ -138,6 +138,7 @@ class AsciidoctorConventions {
 
 	private void configureHtmlOnlyAttributes(AbstractAsciidoctorTask asciidoctorTask) {
 		Map<String, Object> attributes = new HashMap<>();
+		attributes.put("source-highlighter", "highlightjs");
 		attributes.put("highlightjsdir", "js/highlight");
 		attributes.put("highlightjs-theme", "github");
 		attributes.put("linkcss", true);

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/asciidoc/index.adoc

@@ -3,7 +3,6 @@ Andy Wilkinson
 :doctype: book
 :toc: left
 :toclevels: 4
-:source-highlighter: prettify
 :numbered:
 :icons: font
 :hide-uri-scheme:

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/index.adoc

@@ -3,7 +3,6 @@ Andy Wilkinson, Scott Frederick
 :doctype: book
 :toc: left
 :toclevels: 4
-:source-highlighter: prettify
 :numbered:
 :icons: font
 :hide-uri-scheme:

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/asciidoc/index.adoc

@@ -3,7 +3,6 @@ Stephane Nicoll, Andy Wilkinson, Scott Frederick
 :doctype: book
 :toc: left
 :toclevels: 4
-:source-highlighter: prettify
 :numbered:
 :icons: font
 :hide-uri-scheme: