Downgrade AsciidoctorJ 2.3.x testing to use 2.3.0 as 2.3.1 is only partially available from Maven Central

[wilkinsona]

At the time of writing, 2.3.1 on Maven Central is incomplete:

../
asciidoctorj-2.3.1-bin.zip                        2020-06-17 08:05  51357725      
asciidoctorj-2.3.1-javadoc.jar                    2020-06-17 08:05    686532      
asciidoctorj-2.3.1.pom.asc                        2020-06-17 08:05       543 

This causes the build to fail. It's not clear if the files were originally on Maven Central and have since been deleted or if the build was resolving them from elsewhere. All of 2.3.1 is available from JCenter so the latter is possible prior to the tightening of permissions on repo.spring.io.