[serverless] Use current attributes in serverless docs (#665)

Dependent on #666

🍷  Pairs with elastic/security-docs#6577

Use `current` attributes in serverless docs to avoid 404s to `master`
pages.

Author: colleenmcginnis

serverless/index.asciidoc

@@ -1,6 +1,6 @@
 :doctype: book
 
-include::{asciidoc-dir}/../../shared/versions/stack/master.asciidoc[]
+include::{asciidoc-dir}/../../shared/versions/stack/current.asciidoc[]
 include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 
 :security-serverless:         {security-docs-root}/docs/serverless