adjust Asciidoctor plugin to new Gradle version
adjust Asciidoctor plugin to new Gradle version

This was overlooked when we upgraded to Gradle 7. The old plugin is not compatible anymore.

Signed-off-by: Peter Gafert <cdbfcea01cf62f959f8b10da948a53f2869d9b6c@tngtech.com>

TNG/ArchUnit,TNG/ArchUnit