[ci skip] remove guide section on models in migrations

Both examples given are incorrect, and no one can seem to come up with
a realistic situation where this is an actual problem. For now, it seems like
the best thing to do is just scrap the section altogether.
Merge pull request #14208 from lukesteensen/remove-migration-example

[ci skip] remove guide section on models in migrations [ci skip]
move PostgreSQL UUID tests from `datatype_test.rb` to `uuid_test.rb`.
extract common code in `uuid_test.rb`.