The CI script to install dependencies for the documentation build
has been updated to install asciidoctor directly via the system
package manager instead of pinning to an older version via gem.
Additionally, an obsolete variable used for retired Azure Pipelines
environments has been removed.
* jk/ci-use-system-asciidoctor:
ci: drop ALREADY_HAVE_ASCIIDOCTOR variable
ci: fix missing Ruby dependency in "documentation" job
e7621b4bad