Enable master variants with associated order cycles

There's only 5 in UK prod. keeping them is easier than figuring out if it's safe to delete.

(cherry picked from commit 4bf65e330b)

db/migrate/20230605133804_remove_master_variants.rb

@@ -5,6 +5,7 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
     end
 
     handle_master_line_items
+    handle_master_exchange_variants
     delete_master_inventory_units
     delete_master_variant_prices
     delete_master_stock_items
@@ -24,6 +25,17 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
     )
   end
 
+  def handle_master_exchange_variants
+    ActiveRecord::Base.connection.execute(<<-SQL
+      UPDATE spree_variants
+      SET is_master = false
+      FROM exchange_variants
+      WHERE spree_variants.is_master = true
+        AND spree_variants.id = exchange_variants.variant_id
+    SQL
+    )
+  end
+
   def delete_master_inventory_units
     ActiveRecord::Base.connection.execute(<<-SQL
       DELETE FROM spree_inventory_units