Enable :import_graceful_failures by default

We tested this with a large import and it's a pretty safe change

lib/gitlab/import_export/project_tree_restorer.rb

@@ -105,7 +105,7 @@ module Gitlab
         save_id_mapping(relation_key, data_hash, relation_object)
       rescue => e
         # re-raise if not enabled
-        raise e unless Feature.enabled?(:import_graceful_failures, @project.group)
+        raise e unless Feature.enabled?(:import_graceful_failures, @project.group, default_enabled: true)
 
         log_import_failure(relation_key, relation_index, e)
       end