[DOCS] Switch to standard ESS trial attribute (#901)

Author: Nik Richers

docs/en/getting-started/get-started-stack.asciidoc

@@ -50,7 +50,7 @@ data, such as logs or decoded network packets.
 Elasticsearch can be run on your own hardware or using our hosted
 Elasticsearch Service on https://www.elastic.co/cloud[Elastic Cloud], which is
 available on AWS and GCP. You can
-https://www.elastic.co/cloud/elasticsearch-service/signup[try out the Elasticsearch Service] for free.
+{ess-trial}[try out the Elasticsearch Service] for free.
 ==========
 
 To download and install {es}, open a terminal window and use the commands that

docs/en/infraops/installation.asciidoc

@@ -15,8 +15,7 @@ license. To learn how to get started quickly, see
 You can skip having to install {es} and {kib} by using our
 https://www.elastic.co/cloud/elasticsearch-service[hosted {es} Service] on
 Elastic Cloud. The {es} Service is available on both AWS and GCP.
-https://www.elastic.co/cloud/elasticsearch-service/signup[Try out the {es}
-Service for free].
+{ess-trial}[Try out the {es} Service for free].
 ==============
 
 * {beats} shippers (version 6.5 or later) installed on each system you want to

docs/en/install-upgrade/installing-stack.asciidoc

@@ -31,4 +31,5 @@ Elastic Cloud is the official hosted Elasticsearch and Kibana offering from Elas
 
 Some Elastic Cloud features can be used only with a specific  link:https://www.elastic.co/cloud/as-a-service/subscriptions[subscription level]. For example, installing custom plugins, dictionaries, and scripts requires a Gold or Platinum subscription.
 
-To learn more, see {cloud-ref}/getting-started.html[Getting Started with Elastic Cloud]. Or just link:https://www.elastic.co/cloud/as-a-service/signup[sign up for a free trial] and start exploring hosted Elasticsearch in minutes.
+To learn more, see {cloud-ref}/getting-started.html[Getting Started with Elastic Cloud].
+Or just {ess-trial}[sign up for a free trial] and start exploring hosted Elasticsearch in minutes.