Use the new postgres-version argument from action-setup-postgres

[action-setup-postgres v7] introduced a `postgres-version` argument.

We should incorporate this, because the action uses postgres 18 by default, which is not yet (officially) supported by some of the dependencies in e.g. FDP.


[action-setup-postgres v7]: https://github.com/ikalnytskyi/action-setup-postgres/releases/tag/v7