There's a bug in some versions of AsciidoctorJ where extensions are
only called on this first conversion performed by a given Asciidoctor
instance. This affects the multi-page documentation where there are
multiple conversions performed – one for each page in the
documentation.

This commit upgrades the version of AsciidoctorJ used by the
Asciidoctor Gradle plugin to 2.4.1, which contains fix for the problem.

Closes gh-23709