fix: CI failing due to destination database PG16 version

[k3yss]

This PR updates the destination database version from 16 to 15. According to the documentation:

If the database version for your instance is PostgreSQL 16 or later, the default edition is Cloud SQL Enterprise Plus.

However, the current CI configuration assumes our edition is Enterprise resulting in a failure due to Enterprise Plus not recognizing the custom configuration syntax.

cc: https://cloud.google.com/sql/docs/postgres/create-instance#machine-types