[Upgrade 9.0] Adds Enterprise stub page (#1023)

Resolves elastic/docs-projects#477.

Previews: 
- [Upgrade landing
page](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1023/deploy-manage/upgrade)
- [Upgrade to Enterprise
Search](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1023/deploy-manage/upgrade/deployment-or-cluster/enterprise-search)

Author: jmikell821

deploy-manage/toc.yml

@@ -798,7 +798,8 @@ toc:
               - file: upgrade/deployment-or-cluster/kibana.md
                 children:
                   - file: upgrade/deployment-or-cluster/saved-object-migrations.md
-                  - file: upgrade/deployment-or-cluster/kibana-roll-back.md    
+                  - file: upgrade/deployment-or-cluster/kibana-roll-back.md
+          - file: upgrade/deployment-or-cluster/enterprise-search.md   
       - file: upgrade/ingest-components.md        
   - file: uninstall.md
     children:

deploy-manage/upgrade.md

@@ -2,6 +2,10 @@
 
 Upgrading to the latest version provides access to the newest Elastic features, enhancements, performance improvements, and bug fixes. These updates reduce costs, speed up threat response, and improve investigative and analytical data tools. 
 
+:::{important}
+In {{stack}} 9.0.0 and beyond, Enterprise Search is unavailable. For more information, refer to [Migrating to 9.x from Enterprise Search 8.x versions](https://www.elastic.co/guide/en/enterprise-search/8.18/upgrading-to-9-x.html).
+:::
+
 When Elastic releases new versions, older versions reach their end of life on a set schedule. To keep your deployment supported, stay up to date. For more information, refer to [Product End of Life Dates](https://www.elastic.co/support/eol).
 
 :::{note}

deploy-manage/upgrade/deployment-or-cluster/enterprise-search.md

@@ -0,0 +1,15 @@
+---
+applies_to:
+  stack:
+  deployment:
+    self:
+    eck:
+    ess:
+    ece:
+---
+
+# Upgrade to Enterprise Search
+
+In {{stack}} 9.0.0 and beyond, Enterprise Search is unavailable. We recommend migrating to {{es}} for improved performance, scalability, and flexibility. For information about what {{es}} features you can use, check out this [blog post](https://www.elastic.co/blog/app-search-to-elasticsearch). 
+
+For complete information about resources and initiatives related to App Search migrations, future enhancements, and decommissioning, refer to [Migrating to 9.x from Enterprise Search 8.x versions](https://www.elastic.co/guide/en/enterprise-search/8.18/upgrading-to-9-x.html).