Set master variants which are associated to line items to non-master

Line items which reference a master variant is a scenario that in theory shouldn't have been valid or even possible for at least 5-6 years, and these old bits of data in theory should have been cleaned up at the time those changes were made. But a couple of servers have some really old data that's not in a nice state.

Here we can just flip the is_master flag to false for those specific (legacy data) cases before deleting any other master variants, to keep the legacy line item data intact.

db/migrate/20230605133804_remove_master_variants.rb

@@ -4,7 +4,7 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
       delete_master_option_values
     end
 
-    delete_master_line_items
+    handle_master_line_items
     delete_master_inventory_units
     delete_master_variant_prices
     delete_master_variants
@@ -12,10 +12,11 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
 
   private
 
-  def delete_master_line_items
+  def handle_master_line_items
     ActiveRecord::Base.connection.execute(<<-SQL
-      DELETE FROM spree_line_items
-      USING spree_variants
+      UPDATE spree_variants
+      SET is_master = false
+      FROM spree_line_items
       WHERE spree_variants.is_master = true
         AND spree_variants.id = spree_line_items.variant_id
     SQL