Replaces instance of STS with attribute

I am interested in seeing how this edit feature works. Eric recently merged STS attributes into `main`, so I thought this would be a reasonable way to try this out.

Author: jeana-redhat

modules/rosa-sts-overview-of-the-default-cluster-specifications.adoc

@@ -26,7 +26,7 @@ endif::rosa-hcp[]
 ifndef::rosa-hcp[]
 {product-title} (ROSA)
 endif::rosa-hcp[]
-cluster with the AWS Security Token Service (STS) by using the default installation options. The following summary describes the default cluster specifications.
+cluster with the {sts-first} by using the default installation options. The following summary describes the default cluster specifications.
 endif::rosa-terraform[]
 
 ifdef::rosa-hcp[]
@@ -146,4 +146,4 @@ ifeval::["{context}" == "rosa-sts-creating-a-cluster-quickly"]
 endif::[]
 ifeval::["{context}" == "rosa-sts-creating-a-cluster-quickly-terraform"]
 :!rosa-terraform:
-endif::[]
+endif::[]