Revert "Upgrade to asciidoctor-maven-plugin 1.6.0"

This commit introduced an incompatible change in the asciidoct
API: both asciidoctorj-pdf and spring-asciidoctor-extensions
expect `org.asciidoctor.extension.JavaExtensionRegistry` to be
a class, not an interface.

This reverts commit 120ffb1ed0.

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

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