Enable project ndjson ff by default

- Enable project_import_ndjson by default
- Enabled project_export_as_ndjson by default

ee/changelogs/unreleased/216959-enable-project-ndjson-feature-flags.yml

+---
+title: Use data-interchange format based on .ndjson for Project import and export.
+merge_request: 31601
+author:
+type: performance

ee/spec/lib/ee/gitlab/import_export/project/tree_saver_spec.rb

@@ -38,9 +38,13 @@ describe Gitlab::ImportExport::Project::TreeSaver do
     let_it_be(:exportable_path) { 'project' }
 
     before_all do
-      Feature.enable(:project_export_as_ndjson) if ndjson_enabled
-      project.add_maintainer(user)
-      expect(project_tree_saver.save).to be true
+      RSpec::Mocks.with_temporary_scope do
+        allow(Feature).to receive(:enabled?) { true }
+        stub_feature_flags(project_export_as_ndjson: ndjson_enabled)
+        project.add_maintainer(user)
+
+        expect(project_tree_saver.save).to be true
+      end
     end
 
     let_it_be(:issue_json) { get_json(full_path, exportable_path, :issues, ndjson_enabled).first }

lib/gitlab/import_export/project/tree_restorer.rb

@@ -54,7 +54,7 @@ module Gitlab
         end
 
         def ndjson_relation_reader
-          return unless Feature.enabled?(:project_import_ndjson, project.namespace)
+          return unless Feature.enabled?(:project_import_ndjson, project.namespace, default_enabled: true)
 
           ImportExport::JSON::NdjsonReader.new(
             File.join(shared.export_path, 'tree')

lib/gitlab/import_export/project/tree_saver.rb

@@ -54,7 +54,7 @@ module Gitlab
 
         def json_writer
           @json_writer ||= begin
-            if ::Feature.enabled?(:project_export_as_ndjson, @project.namespace)
+            if ::Feature.enabled?(:project_export_as_ndjson, @project.namespace, default_enabled: true)
               full_path = File.join(@shared.export_path, 'tree')
               Gitlab::ImportExport::JSON::NdjsonWriter.new(full_path)
             else