Always configure PDF backend as task will only run on cache miss

Previously, the Asciidoctor task was not cacheable and generating the
PDF documentation was very slow. To improve build times, the PDF
documentation was not generated for snapshot builds.

The upgrade to 2.4.0 of the Asciidoctor Gradle pluging means that the
Asciidoctor task is now cacheable. As such, its tasks will only run
when the documentation has changed. This should allow PDF
documentation to be published for every build without slowing things
down too much and the cost of generating the documentation will only
be incurred when there is a change to the documentation.

See gh-24216

Author: wilkinsona

gradle/docs.gradle

@@ -128,11 +128,7 @@ asciidoctor {
 	}
 	logDocuments = true
 	outputOptions {
-		backends = ["html5"]
-		// only ouput PDF documentation for non-SNAPSHOT builds
-		if (!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
-			backends += "pdf"
-		}
+		backends = ["html5", "pdf"]
 	}
 	options doctype: 'book', eruby: 'erubis'
 	attributes([