Enable default fields for Elasticsearch 6.x

Previously default_fields were only enabled for Elasticsearch 7.x. For the migration it is required that also indices from Elasticsearch 6.x have the default_field option set. This enables it for 6.x.

Closes elastic#11187

libbeat/template/template.go

@@ -277,7 +277,7 @@ func buildIdxSettings(ver common.Version, userSettings common.MapStr) common.Map
 		indexSettings.Put("number_of_routing_shards", defaultNumberOfRoutingShards)
 	}
 
-	if ver.Major >= 7 {
+	if ver.Major >= 6 {
 		// copy defaultFields, as defaultFields is shared global slice.
 		fields := make([]string, len(defaultFields))
 		copy(fields, defaultFields)