Use Rouge syntax highlighting in PDF version of Reference Manual

This commit uses a tip provided by @mojavelinux to set the syntax
highlighter to Rouge for the PDF version of the User Guide.

Once Asciidoctor 1.5.7 has been released we will likely switch to Rouge
for the HTML version of the Reference Manual as well.

Issue: SPR-14997

gradle/docs.gradle

@@ -30,7 +30,7 @@ asciidoctor {
 				revnumber: project.version,
 				sectanchors: '',
 				sectnums: '',
-				'source-highlighter': 'coderay',
+				'source-highlighter': 'coderay@', // TODO switch to 'rouge' once supported by the html5 backend
 				stylesdir: 'stylesheets/',
 				stylesheet: 'spring.css',
 				'spring-version': project.version

src/docs/asciidoc/appendix.adoc

@@ -5,6 +5,7 @@
 :wiki-spring-framework: https://github.com/spring-projects/spring-framework/wiki
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 [[spring-whats-new]]
 == What's New in the Spring Framework

src/docs/asciidoc/core.adoc

@@ -4,6 +4,7 @@
 :api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 This part of the reference documentation covers all of those technologies that are
 absolutely integral to the Spring Framework.

src/docs/asciidoc/data-access.adoc

@@ -4,6 +4,7 @@
 :api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 This part of the reference documentation is concerned with data access and the
 interaction between the data access layer and the business or service layer.

src/docs/asciidoc/integration.adoc

@@ -6,6 +6,7 @@
 :doc-spring-gemfire: {doc-root}/spring-gemfire/docs/current/reference
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 This part of the reference documentation covers the Spring Framework's integration with
 a number of Java EE (and related) technologies.

src/docs/asciidoc/overview.adoc

@@ -4,6 +4,7 @@
 :api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 
 [[overview-getting-started-with-spring]]

src/docs/asciidoc/testing.adoc

@@ -5,6 +5,7 @@
 :doc-spring-boot: {doc-root}/spring-boot/docs/current/reference
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 The adoption of the test-driven-development (TDD) approach to software
 development is certainly advocated by the Spring team, and so coverage of Spring's

src/docs/asciidoc/web.adoc

@@ -4,6 +4,7 @@
 :api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework
 :toc: left
 :toclevels: 2
+ifdef::backend-pdf[:source-highlighter: rouge]
 
 This part of the reference documentation covers Spring Framework's support for the
 presentation tier (and specifically web-based presentation tiers) including support