Enable diff batch loading feature flag by default

Closes https://gitlab.com/gitlab-org/gitlab/issues/38106

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -18,7 +18,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   end
 
   def diffs_batch
-    return render_404 unless Feature.enabled?(:diffs_batch_load, @merge_request.project)
+    return render_404 unless Feature.enabled?(:diffs_batch_load, @merge_request.project, default_enabled: true)
 
     diffs = @compare.diffs_in_batch(params[:page], params[:per_page], diff_options: diff_options)
     positions = @merge_request.note_positions_for_paths(diffs.diff_file_paths, current_user)

app/controllers/projects/merge_requests_controller.rb

@@ -19,7 +19,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   before_action :authenticate_user!, only: [:assign_related_issues]
   before_action :check_user_can_push_to_source_branch!, only: [:rebase]
   before_action only: [:show] do
-    push_frontend_feature_flag(:diffs_batch_load, @project)
+    push_frontend_feature_flag(:diffs_batch_load, @project, default_enabled: true)
     push_frontend_feature_flag(:single_mr_diff_view, @project)
     push_frontend_feature_flag(:suggest_pipeline) if experiment_enabled?(:suggest_pipeline)
   end