Add _field_names disabling to archival index tests (#117703)

Disabling the "_field_names" field in mappings was possible until 8.x
and now issues a deprecation warning. We need to maintain the ability to
read these mappings for archival indices so this change adds this case
to one of the index mappings in tests and checks for the deprecation
warning for it.

Author: cbuescher

x-pack/qa/repository-old-versions/src/test/java/org/elasticsearch/oldrepos/OldMappingsIT.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.Version;
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.RequestOptions;
+import org.elasticsearch.client.Response;
 import org.elasticsearch.client.ResponseException;
 import org.elasticsearch.client.RestClient;
 import org.elasticsearch.client.WarningsHandler;
@@ -166,7 +167,10 @@ public void setupIndex() throws IOException {
         createRestoreRequest.addParameter("wait_for_completion", "true");
         createRestoreRequest.setJsonEntity("{\"indices\":\"" + indices.stream().collect(Collectors.joining(",")) + "\"}");
         createRestoreRequest.setOptions(RequestOptions.DEFAULT.toBuilder().setWarningsHandler(WarningsHandler.PERMISSIVE));
-        assertOK(client().performRequest(createRestoreRequest));
+        Response response = client().performRequest(createRestoreRequest);
+        // check deprecation warning for "_field_name" disabling
+        assertTrue(response.getWarnings().stream().filter(s -> s.contains("Disabling _field_names is not necessary")).count() > 0);
+        assertOK(response);
     }
 
     private Request createIndex(String indexName, String file) throws IOException {

x-pack/qa/repository-old-versions/src/test/resources/org/elasticsearch/oldrepos/custom.json

@@ -1,4 +1,7 @@
 "_default_": {
+  "_field_names": {
+    "enabled": false
+  },
   "properties": {
     "apache2": {
       "properties": {