docs: deprecation notice on state api (#3561)

## About the changes
Update documentation about the deprecation of state API.

Related to #1265

website/docs/reference/api/legacy/unleash/admin/state.md

@@ -3,6 +3,12 @@ id: state
 title: /api/admin/state
 ---
 
+:::caution Deprecation notice
+
+Api admin state is deprecated from version 5. We recommend using `/api/admin/features-batch` instead for [import](https://docs.getunleash.io/reference/api/unleash/import-toggles) and [export](https://docs.getunleash.io/reference/api/unleash/export-features).
+
+:::
+
 > In order to access the admin API endpoints you need to identify yourself. Unless you're using the `none` authentication method, you'll need to [create an ADMIN token](/how-to/how-to-create-api-tokens) and add an Authorization header using the token.
 
 ### Export Feature Toggles & Strategies {#export-feature-toggles--strategies}

website/docs/reference/deploy/import-export.md

@@ -1,5 +1,5 @@
 ---
-title: Import & Export
+title: '[Deprecated] Import & Export'
 ---
 import ApiRequest from '@site/src/components/ApiRequest'
 
@@ -9,6 +9,12 @@ The import and export API first appeared in Unleash 3.3.0.
 
 :::
 
+:::caution Deprecation notice
+
+Api admin state is deprecated from version 5. We recommend using `/api/admin/features-batch` instead for [import](https://docs.getunleash.io/reference/api/unleash/import-toggles) and [export](https://docs.getunleash.io/reference/api/unleash/export-features).
+
+:::
+
 Unleash supports import and export of feature toggles and strategies at startup and during runtime. The main purpose of the import/export feature is to bootstrap new Unleash instances with feature toggles and their configuration. If you are looking for a granular way to keep seperate Unleash instances in sync we strongly recommend that you take a look at the Unleash Admin APIs.
 
 The import mechanism guarantees that: