Upgrade to asciidoctor-maven-plugin 1.6.0

Author: snicoll

spring-boot-project/spring-boot-parent/pom.xml

@@ -293,14 +293,7 @@
 				<plugin>
 					<groupId>org.asciidoctor</groupId>
 					<artifactId>asciidoctor-maven-plugin</artifactId>
-					<version>1.5.7.1</version>
-					<dependencies>
-						<dependency>
-							<groupId>org.asciidoctor</groupId>
-							<artifactId>asciidoctorj</artifactId>
-							<version>1.5.8</version>
-						</dependency>
-					</dependencies>
+					<version>1.6.0</version>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>