Add shared attributes

Author: lcawl

serverless/pages/serverless-changelog.asciidoc

@@ -1,5 +1,5 @@
-Review the https://www.elastic.co/guide/en/elastic-stack/9.0/serverless-changelog.html[latest improvements and changes to Elastic Cloud Serverless].
+Review the {stack-ref-90}/serverless-changelog.html[latest improvements and changes to Elastic Cloud Serverless].
 
 For serverless API changes, refer to https://www.elastic.co/docs/api/changes[APIs Changelog].
 
-For serverless changes in Cloud Console, refer to https://www.elastic.co/guide/en/cloud/current/ec-release-notes.html[Elasticsearch Service Documentation: Release notes].
+For serverless changes in Cloud Console, refer to {cloud}/ec-release-notes.html[Elasticsearch Service Documentation: Release notes].