deprecate annotation discovery

Closes #3521472

core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php

@@ -14,6 +14,11 @@
 
 /**
  * Defines a discovery mechanism to find annotated plugins in PSR-4 namespaces.
+ *
+ * @deprecated in drupal:11.2.0 and is removed from drupal:12.0.0. Use attribute
+ *   discovery instead.
+ *
+ * @see https://www.drupal.org/node/3395582
  */
 class AnnotatedClassDiscovery implements DiscoveryInterface {
 
@@ -77,6 +82,7 @@ public function __construct($plugin_namespaces = [], $plugin_definition_annotati
     $file_cache_suffix = str_replace('\\', '_', $plugin_definition_annotation_name);
     $file_cache_suffix .= ':' . Crypt::hashBase64(serialize($annotation_namespaces));
     $this->fileCache = FileCacheFactory::get('annotation_discovery:' . $file_cache_suffix);
+    @trigger_error('Annotation discovery is deprecated in drupal:11.2.0 and is removed from drupal:12.0.0. Use attribute discovery instead. See https://www.drupal.org/node/3395582', E_USER_DEPRECATED);
   }
 
   /**