Enable Rubocop Style/FrozenStringLiteralComment (#23793)

db/post_migrate/20190519130537_remove_boosts_widening_audience.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RemoveBoostsWideningAudience < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 

db/post_migrate/20210308133107_remove_subscription_expires_at_from_accounts.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RemoveSubscriptionExpiresAtFromAccounts < ActiveRecord::Migration[5.2]
   def change
     safety_assured do

db/post_migrate/20220118183123_remove_rememberable_from_users.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RemoveRememberableFromUsers < ActiveRecord::Migration[6.1]
   def change
     safety_assured do