Use allowed_warnings instead or warnings

Author: gmarouli

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/deprecation/10_basic.yml

@@ -11,10 +11,10 @@ setup:
         - method: GET
           path: /_migration/deprecations
           capabilities: [ data_streams, ilm_policies, templates ]
-      test_runner_features: [capabilities, warnings]
+      test_runner_features: [capabilities, allowed_warnings]
       reason: "Support for data streams, ILM policies and templates"
   - do:
-      warnings:
+      allowed_warnings:
         - "this request accesses system indices: [.security-7], but in a future major version, direct access to system indices will be prevented by default"
       migration.deprecations:
         index: "*"