chore(deps): update docker.io/library/postgres docker tag to v18 - #320

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: peter-evans/create-pull-request@v7. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/