Include io.spring.asciidoctor.backends group

Update `repositories` declaration to include the
'io.spring.asciidoctor.backends' group.

See gh-26293

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

@@ -22,6 +22,7 @@ repositories {
 		url "https://repo.spring.io/release"
 		mavenContent {
 			includeGroup "io.spring.asciidoctor"
+			includeGroup "io.spring.asciidoctor.backends"
 		}
 	}
 }