asciidoctor: 1.5.7.1 -> 2.0.10

There has been a new major version, so it seems good to upgrade.

I had to drop two plugins:
- `asciidoctor-latex` seems dead, and hasn't had any commits since 2015.
- `asciidoctor-bespoke` also hasn't been updated for over a year, and
doesn't seem to be compatible with the new version.