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-actuator-autoconfigure/build.gradle

@@ -18,6 +18,7 @@ repositories {
 		url "https://repo.spring.io/release"
 		mavenContent {
 			includeGroup "io.spring.asciidoctor"
+			includeGroup "io.spring.asciidoctor.backends"
 		}
 	}
 }

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"
 		}
 	}
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/build.gradle

@@ -18,6 +18,7 @@ repositories {
 		url "https://repo.spring.io/release"
 		mavenContent {
 			includeGroup "io.spring.asciidoctor"
+			includeGroup "io.spring.asciidoctor.backends"
 		}
 	}
 }

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/build.gradle

@@ -12,6 +12,16 @@ configurations {
 	documentation
 }
 
+repositories {
+	maven {
+		url "https://repo.spring.io/release"
+		mavenContent {
+			includeGroup "io.spring.asciidoctor"
+			includeGroup "io.spring.asciidoctor.backends"
+		}
+	}
+}
+
 dependencies {
 	compileOnly("org.apache.maven.plugin-tools:maven-plugin-annotations")
 	compileOnly("org.sonatype.plexus:plexus-build-api")