Apparently there are some breaking changes with the next major release of Antora. See here for some discussion about what has changed.
It looks like what needs to be updated is pretty trivial. I think we can just follow darknao's lead in the below PR and make similar changes to our build.sh and site.yml.
build.sh
site.yml
https://pagure.io/fedora-docs/docs-fp-o/pull-request/178#request_diff
Closed by #56
Metadata Update from @glb: - Issue status updated to: Closed (was: Open)