Update rest-api-spec (#2024)

Author: github-actions[bot]

specification/_json_spec/knn_search.json

@@ -5,6 +5,10 @@
       "description": "Performs a kNN search."
     },
     "stability": "experimental",
+    "deprecated": {
+      "version": "8.4.0",
+      "description": "The kNN search API has been replaced by the `knn` option in the search API."
+    },
     "visibility": "public",
     "headers": {
       "accept": ["application/json"],