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

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

ci/install-dependencies.sh

@@ -164,9 +164,8 @@ sparse)
 		libexpat-dev gettext zlib1g-dev sparse
 	;;
 Documentation)
-	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make
+	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby
 
-	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor
 	sudo gem install concurrent-ruby
 	;;