Configure the Asciidoctor revnumber in a central location

See gh-20934

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

@@ -155,10 +155,6 @@ tasks.withType(org.asciidoctor.gradle.jvm.AbstractAsciidoctorTask) {
 	}
 }
 
-asciidoctor {
-	attributes "revnumber": null
-}
-
 asciidoctor {
 	sources {
 		include "*.singleadoc"
@@ -171,20 +167,12 @@ asciidoctorPdf {
 	}
 }
 
-asciidoctorPdf {
-	attributes "revnumber": null
-}
-
 task asciidoctorMultipage(type: org.asciidoctor.gradle.jvm.AsciidoctorTask) {
 	sources {
 		include "*.adoc"
 	}
 }
 
-asciidoctorMultipage {
-	attributes "revnumber": null
-}
-
 syncDocumentationSourceForAsciidoctor {
 	dependsOn documentTestSlices
 	dependsOn documentStarters