Deprecate special handling of ReindexRequest#size

Author: arteam

modules/reindex/build.gradle

@@ -167,3 +167,7 @@ if (OS.current() == OS.WINDOWS) {
     }
   }
 }
+
+tasks.named("yamlRestCompatTestTransform").configure { task ->
+  task.skipTest("reindex/20_validation/specifying size fails", "size is rejected in 9.0")
+}

server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java

@@ -354,10 +354,6 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         );
 
         PARSER.declareInt(ReindexRequest::setMaxDocsValidateIdentical, new ParseField("max_docs"));
-
-        // avoid silently accepting an ignored size.
-        PARSER.declareInt((r, s) -> failOnSizeSpecified(), new ParseField("size"));
-
         PARSER.declareField((p, v, c) -> v.setScript(Script.parse(p)), new ParseField("script"), ObjectParser.ValueType.OBJECT);
         PARSER.declareString(ReindexRequest::setConflicts, new ParseField("conflicts"));
     }
@@ -497,8 +493,4 @@ static void setMaxDocsValidateIdentical(AbstractBulkByScrollRequest<?> request,
             request.setMaxDocs(maxDocs);
         }
     }
-
-    private static void failOnSizeSpecified() {
-        throw new IllegalArgumentException("invalid parameter [size], use [max_docs] instead");
-    }
 }