docs: add notice linking to analysis_server_plugin

Closes #61820

GitOrigin-RevId: f45bed0
Change-Id: I77d6708b9a52fe40ed81a9e99e12b7df4ab01f2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/457580
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: Hassnaa9

pkg/analyzer_plugin/README.md

@@ -3,6 +3,13 @@
 
 A framework for building plugins for the analysis server.
 
+> **WARNING!**  
+> This package is for **legacy support** and is **not recommended** for new plugin development.  
+>  
+> For modern plugin implementations, use the [`analysis_server_plugin`](https://pub.dev/packages/analysis_server_plugin) package instead.  
+>  
+> See its documentation for the latest architecture and examples.
+
 ## Usage
 
 **Note:** The plugin support is not currently available for general use.