ci: roll PG matrix to 16/17/18 and bump min PG to 16

Suleimi Ahmedrequested to merge
sahmed/pg18-ci-matrix into master

Summary

Prep work for the upcoming production PostgreSQL 17 → 18 major-version cutover. This MR rolls the CI matrix forward by one major and raises the minimum supported PostgreSQL version in lockstep:

  • .gitlab-ci.yml: PG_PREV=16, PG_CURR=17, PG_NEXT=18 (was 15/16/17).
  • registry/datastore/datastore.go: MinPostgresqlVersion bumped to "16.0".
  • registry/datastore/db_internal_test.go: mock query thresholds and case comments updated to match the new floor (>= 160000).
  • docs/database-local-setup.md: minimum-required note updated to PG 16.

The integration check at .gitlab/ci/integration.yml:391-399 asserts PG_PREV_VERSION == MinPostgresqlVersion; both values are bumped together so the check stays green.

The postgresql-ci:18 image has been published to registry.gitlab.com/gitlab-org/container-registry/postgresql-ci:18 so the PG_NEXT matrix slot has an image to pull.

Edited by Suleimi Ahmed

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