fix(ci): pin pr-test-gate Postgres version to match ci.yml and docs - #1387

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Unexpected input(s) 'workspace', valid inputs are ['prefix-key', 'shared-key', 'key', 'add-job-id-key', 'add-rust-environment-hash-key', 'env-vars', 'workspaces', 'cache-directories', 'cache-targets', 'cache-on-failure', 'cache-all-crates', 'cache-workspace-crates', 'save-if', 'cache-provider', 'cache-bin', 'lookup-only', 'cmd-format']