Use GitHub admonition for browser support warning (#1997)

Author: pquentin

README.md

@@ -73,7 +73,8 @@ npm install @elastic/elasticsearch@<major>
 
 #### Browser
 
-WARNING: There is no official support for the browser environment. It exposes your Elasticsearch instance to everyone, which could lead to security issues.
+> [!WARNING]
+> There is no official support for the browser environment. It exposes your Elasticsearch instance to everyone, which could lead to security issues.
 We recommend that you write a lightweight proxy that uses this client instead, you can see a proxy example [here](./docs/examples/proxy).
 
 ## Documentation