Add tests for PG 15 and 16

[damacus]

As per end of life docs https://endoflife.date/postgresql

We've now deprecated PG11, but we should add support for PG15 and 16

[jimdaga]

Hi @damacus - My team is looking to start testing PG16, and I noticed 16 isn't a supported version yet.

I could look to submit a PR to add those changes, but given it's been a few years since I've looked at this code do you happen to have any previous PRs I could reference?

One thing we've noticed;
We're going to have to remove stats_temp_directory from createcluster.conf on pg16+ as that's no longer supported (link)