Switch to spring-asciidoctor-backends SNAPSHOT

See gh-26293

buildSrc/src/main/java/org/springframework/boot/build/AsciidoctorConventions.java

@@ -97,7 +97,7 @@ class AsciidoctorConventions {
 		ConfigurationContainer configurations = project.getConfigurations();
 		Configuration asciidoctorExtensions = configurations.maybeCreate(EXTENSIONS_CONFIGURATION);
 		asciidoctorExtensions.getDependencies().add(project.getDependencies()
-				.create("io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.1-M1"));
+				.create("io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.1-SNAPSHOT"));
 		Configuration dependencyManagement = configurations.findByName("dependencyManagement");
 		if (dependencyManagement != null) {
 			asciidoctorExtensions.extendsFrom(dependencyManagement);