[docs] Document internal API restriction in 9.0 (#191943)

fix: https://github.com/elastic/kibana/issues/191941

Adds note that access to internal APIs will be restricted as of 9.0:


![document-restricted-internal-API-access](https://github.com/user-attachments/assets/56b5c43d-ff6f-4b5b-8c18-2cbe1a1de7c3)

- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
Co-authored-by: lcawl <lcawley@elastic.co>

docs/user/api.asciidoc

@@ -5,6 +5,8 @@ Some {kib} features are provided via a REST API, which is ideal for creating an
 integration with {kib}, or automating certain aspects of configuring and
 deploying {kib}.
 
+NOTE: Access to internal {kib} API endpoints will be restricted in 9.0. Please move any integrations to publicly documented APIs.
+
 NOTE: For the latest details, refer to {api-kibana}[{kib} API].
 
 [float]

oas_docs/kibana.info.yaml

@@ -24,6 +24,10 @@ info:
     ```
 
     For more information about the console, refer to [Run API requests](https://www.elastic.co/guide/en/kibana/current/console-kibana.html).
+
+    NOTE: Access to internal Kibana API endpoints will be restricted in Kibana version 9.0. Please move any integrations to publicly documented APIs.
+
+    
   version: '1.0.2'
   license:
     name: Attribution-NonCommercial-NoDerivatives 4.0 International
@@ -53,4 +57,4 @@ components:
         For example: `Authorization: ApiKey base64AccessApiKey`
     basicAuth:
       type: http
-      scheme: basic
+      scheme: basic