Showing a warning when PostgreSQL can't disable foreign key constraints

PostgreSQL can't properly disable foreign key constraints when the PostgreSQL user is not superuser. Added a warning when `ActiveRecord::InvalidForeignKey` is throw to make the user aware about the issue.

Reference #17542

---

@senny @matthewd I had technical problems with the other PR (https://github.com/rails/rails/pull/17663) and had to open another one.
should we move the file into the postgresql specific tests and get rid of this guard?