Update tested postgres versions

Currently, we test in MRs against postgres 13, and nightly against 12 and 14.

Since we're going to postgres 14 on gitlab.com, we should make that the primary version we test against, and we should add nightly tests for postgres 15. Although we don't officially support it, we don't prevent it and knowing if there are issues early will help us make sure that we catch any bugs early.

Deliverables

• Swap postgres 14 to be default, and 13 to be nightly
• Add nightly tests for postgres 15

References

We recently added Postgres 14 and swapped 12 and 13 in !114624 (merged), so it should be pretty similar to this effort, but with swapping 13 and 14 and adding 15.