[CI] Buildkite: Retries on 400 and 503 in serverless tests

Author: picandocodigo

elasticsearch-api/spec/yaml-test-runner/run.rb

@@ -62,7 +62,7 @@ def serverless?
 if serverless?
   options.merge!(
     {
-      retry_on_status: [409],
+      retry_on_status: [409, 400, 503],
       retry_on_failure: 10,
       delay_on_retry: 60_000,
       request_timeout: 120