Add TODO to update step that finds closed briefs to use DOS4
Add TODO to update step that finds closed briefs to use DOS4

Currently there are very few DOS4 closed briefs that can be used for
tests, and there are still lots of closed DOS3 briefs. At some point in
the future that will change and we can update our tests; for now we
leave them using DOS3.

alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests