Merge branch 'jk/ci-use-system-asciidoctor' into next

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.

Graduated to 'master'.
cf. <apfWhYF6nmcFGKE3@pks.im>

* jk/ci-use-system-asciidoctor:
  ci: use system asciidoctor

ci/install-dependencies.sh

@@ -164,9 +164,9 @@ sparse)
 		libexpat-dev gettext zlib1g-dev sparse
 	;;
 Documentation)
-	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby
+	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby \
+		asciidoctor
 
-	sudo gem install --version 1.5.8 asciidoctor
 	sudo gem install concurrent-ruby
 	;;
 esac