Maintenance on PostgreSQL clients

[sachilles]

Since PostgreSQL v18 was already published we need to provide a suitable postgresql client as well.

However, following the requirements for GitLab CE and in particular for PostgreSQL (see https://docs.gitlab.com/18.5/install/requirements/#postgresql) we should remove all clients for PostgresSQL versions smaller than 16.

[sachilles]

@kkimurak What do you think about this PR and the removal of old PostgreSQL client versions. I guess, we have to add a comment to the release notes. Otherwise it might be possible that a lot of users could run into trouble.

[kkimurak]

I think it's ok to remove old PostgreSQL clients. In addition to release notes, how about displaying a warning (or terminating with an error) if the minimum and maximum version requirements are not met?
The process for obtaining the server version has already been implemented, so changing it to the following should generally work (see my rough implementation: check-potgresql-server-version-and-warn branch based on this PR, especially commit 7199c08)

By the way, according to the link, Maximum PostgreSQL version for GitLab 18.x is 17.x (tested against GitLab 17.10 and later)

[sachilles]

@kkimurak Nice idea and I'll take over your commits. However, since GitLab does not support PostgreSQL v18 so far not officially, we'll wait before publishing this branch.