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.
note that this code lives in: `activerecord/lib/active_record/connection_adapters/postgresql/referential_integrity.rb` which is specific to the PostgreSQLAdapter. No need to `is_a?` check on the connection.