[Search] Add ML as required plugin to Search Assistant (#204009)

## Summary

This adds the `ml` plugin as required to the Search Assistant so that we
don't need users to navigate to an ml-based plugin to initiate the
knowledge base.

x-pack/plugins/search_assistant/kibana.jsonc

@@ -16,6 +16,7 @@
     "requiredPlugins": [
       "actions",
       "licensing",
+      "ml", // necessary for assistant's use of knowledge base in assistant package
       "observabilityAIAssistant",
       "triggersActionsUi",
       "share"

x-pack/plugins/serverless_search/kibana.jsonc

@@ -38,6 +38,8 @@
       "searchPlayground",
       "usageCollection"
     ],
-    "requiredBundles": ["kibanaReact"]
+    "requiredBundles": [
+      "kibanaReact"
+    ]
   }
 }