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

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@senny @matthewd I had technical problems with the other PR (https://github.com/rails/rails/pull/17663) and had to open another one.
This will warn on every exception... shouldn't it only issue a warning when a foreign key is violated? Otherwise we'll get tons of false positives.