fix(stats): more accurat stats about deleted dossiers

app/tasks/maintenance/backfill_depose_at_on_deleted_dossiers_task.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module Maintenance
+  class BackfillDeposeAtOnDeletedDossiersTask < MaintenanceTasks::Task
+    def collection
+      DeletedDossier.where(depose_at: nil)
+    end
+
+    def process(element)
+      element.update_column(:depose_at, element.deleted_at)
+    end
+  end
+end