Apply backend maintainer suggestions

- Convert the sort argument to string so symbol can be
additionally used.

app/models/milestone.rb

@@ -120,11 +120,11 @@ class Milestone < ApplicationRecord
     sorted.with_order_id_desc
   end
 
-  def self.sort_with_expired_last(sort_by)
+  def self.sort_with_expired_last(method)
     # NOTE: this is a custom ordering of milestones
     # to prioritize displaying non-expired milestones and milestones without due dates
     sorted = reorder(Arel.sql('(CASE WHEN due_date IS NULL THEN 1 WHEN due_date > now() THEN 0 ELSE 2 END) ASC'))
-    sorted = if sort_by == :expired_last_due_date_desc
+    sorted = if method.to_s == 'expired_last_due_date_desc'
                sorted.order(due_date: :desc)
              else
                sorted.order(due_date: :asc)