Add nightly specs for PG18-RC1 and Remove Pgvector

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Overview

With PostgreSQL 18-RC1 released, there are new ways to configure lw locks. We should add specs ensuring we have a basic level of compatibility for PG18. This work is similar to that done in #425470 (closed) and #402982 (closed)

Deliverables

• Add a new Postgres 18 build variant to https://gitlab.com/gitlab-org/gitlab-build-images
• Bump .db-job-base to use PG17.
• Add nightly tests for postgres 18-RC1
• Remove pgvector from the gitlab repo
• Remove pgvector from the gitlab-build-images