Use spring-pdf backend for PDF generation

Update `asciidoctorPdf` tasks to use the `spring-pdf` backend so that
chomped elements and @fold:on/@fold:off comments are hidden.

Closes gh-26373

spring-boot-project/spring-boot-docs/build.gradle

@@ -2,7 +2,6 @@ plugins {
 	id "java"
 	id "groovy"
 	id "org.asciidoctor.jvm.convert"
-	id "org.asciidoctor.jvm.pdf"
 	id "org.springframework.boot.conventions"
 	id "org.springframework.boot.deployed"
 }
@@ -257,7 +256,7 @@ asciidoctor {
 	}
 }
 
-asciidoctorPdf {
+task asciidoctorPdf(type: org.asciidoctor.gradle.jvm.AsciidoctorTask) {
 	sources {
 		include "*.singleadoc"
 	}