Add nightly specs for Postgres 17

Overview

With PostgreSQL 17 released, we should add specs ensuring we have a basic level of compatibility for customers who run external databases who choose to run 17. This work is similar to that done in #425470 (closed) and #402982 (closed)

Deliverables

• Add a new Postgres 16 build variant to https://gitlab.com/gitlab-org/gitlab-build-images
• Bump .db-job-base to use PG16.
• Add nightly tests for postgres 17
• Remove tests for postgres 13