add capabilities

Author: weizijun

server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java

@@ -36,6 +36,9 @@ private SearchCapabilities() {}
     /** knn query where k defaults to the request size. */
     private static final String K_DEFAULT_TO_SIZE = "k_default_to_size";
 
+    /** Support deprecated window_size field in rank. */
+    private static final String RRF_WINDOW_SIZE_SUPPORT_DEPRECATED = "rrf_window_size_support_deprecated";
+
     private static final String RANDOM_SAMPLER_WITH_SCORED_SUBAGGS = "random_sampler_with_scored_subaggs";
     private static final String OPTIMIZED_SCALAR_QUANTIZATION_BBQ = "optimized_scalar_quantization_bbq";
     private static final String KNN_QUANTIZED_VECTOR_RESCORE_OVERSAMPLE = "knn_quantized_vector_rescore_oversample";
@@ -54,6 +57,7 @@ private SearchCapabilities() {}
         capabilities.add(MOVING_FN_RIGHT_MATH);
         capabilities.add(K_DEFAULT_TO_SIZE);
         capabilities.add(KQL_QUERY_SUPPORTED);
+        capabilities.add(RRF_WINDOW_SIZE_SUPPORT_DEPRECATED);
         CAPABILITIES = Set.copyOf(capabilities);
     }
 }

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/100_rank_rrf.yml

@@ -104,10 +104,12 @@ setup:
 "Simple rank with old window_size param":
 
   - requires:
-      cluster_features: ["gte_v8.16.0"]
-      reason: "deprecation added in 8.16"
-      test_runner_features: warnings
-
+      capabilities:
+        - method: POST
+          path: /_search
+          capabilities: [ rrf_window_size_support_deprecated ]
+      test_runner_features: capabilities
+      reason: "add deprecated field [window_size] in 8.18 to support upgrade"
   - do:
       warnings:
         - "Deprecated field [rank] used, replaced by [retriever]"