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.
Do we need a `FakeAdapter`? Would it be possible to manually raise `ActiveRecord::InvalidForeignKey`?

```ruby
@connection.disable_referential_integrity do
  raise ActiveRecord::InvalidForeignKey # simulate missing privileges, foreign keys are still active.
end
```