Add compatibility with maven-site-plugin v3.20.0

[abelsromero]

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What is this issue about?

• Bug report 🪲
• Feature request 🏎
• Question ❓ Consider dropping it first in the Project Chat

Description

As reported in asciidoctor/asciidoctor-maven-examples#297, Asciidoctor Maven site integration v3.0.0 is not compatible with latest maven-site-plugin v3.20.0.
The breaking changes seem to come from using the new major version of Doxia v2.0.0-M12.

Depending on the changes we'll have to decide how the compatibility matrix ends, but it's likely that for simplicity, Asciidoctor modules will only work with maven-site-plugin 3.20.+.

Environment information

• asciidoctor-maven-plugin version: v3.0.0
• asciidoctorj version: _not relevant

• Maven, Java and OS version: not relevant

[abelsromero]

On a side note, I am curious as the reason to integrate non-final version (milestone) of Doxia that introduced breaking changes and do it as a minor version for maven-site-plugin.