Update cross-cluster-search availability (#1204)

CCS is currently incorrectly flagged as available on serverless

h/t @shainaraskas

Author: leemthompo

solutions/search/cross-cluster-search.md

@@ -3,7 +3,7 @@ mapped_pages:
   - https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-cross-cluster-search.html
 applies_to:
   stack:
-  serverless:
+  serverless: unavailable
 ---
 
 # Cross-cluster search [modules-cross-cluster-search]