Update pg build image to 18
What does this MR do and why?
Since PostgreSQL 18-RC1 has been released we should add specs ensuring we have a basic level of compatibility for PG18. This is the first step.
References
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Irina Bronipolsky