Warn on old postgres clients

@tcrayford mentioned that the postgres support team gets a number of tickets caused by people using old versions of postgres clients.

If we know this is an issue we should detect and warn the user and save them some frustration and time.

The issue names no files, tests, or entry points. Start by locating where this Node.js buildpack handles PostgreSQL client dependencies, then determine which client versions are considered old and where a warning could be surfaced; done means users on affected versions receive a clear warning.

Written by the indexing model from the issue text.