Fix failures in vector field usage mixed cluster test

Author: jtibshirani

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/50_dense_vector_field_usage.yml

@@ -53,8 +53,11 @@ setup:
   - skip:
       version: ' - 8.0.99'
       reason: 'dense_vector field usage was added in 8.1'
+      features: ["allowed_warnings"]
   - do:
-      search:
+      allowed_warnings:
+        - "The kNN search API has been replaced by the `knn` option in the search API."
+      knn_search:
         index: futest
         body:
           fields: [ "name" ]