PDF outline only goes two levels deep, making it harder to navigate through extension chapters

Asciidoctor 1.6 (which is currently in use) has an attribute called "toclevels", which sets how many chapter levels will be included in the ToC. It defaults to 2, therefore excluding extension subchapters from both the outline and ToC. While this makes sense for the ToC (which would blow up to even more pages than it already is), it severely limits the outline's usefulness (e.g. to quickly navigate between specific subchapters of extensions).

Asciidoctor 2.0 introduced a separate attribute "outlinelevels" [1]. This would be perfect, however it obviously requires an upgrade to Asciidoctor >= 2.0. I can see there was a problem that prevented the upgrade here, was this ever investigated again? (Or is there a tracking issue to follow?)

[1] https://docs.asciidoctor.org/pdf-converter/2.0/whats-new/#toc-and-pdf-outline

Start with specification/Gemfile around the Asciidoctor 1.6 dependency, then review the referenced openxr.Dockerfile line 55 and the linked upgrade problem. Determine whether the dependency can be upgraded without breaking the documentation build, and verify that extension subchapters appear in the PDF outline while the table of contents remains usable.