refactor: Bump postcss from 8.5.19 to 8.5.28 and css-loader from 7.1.4 to 7.1.5 - #3463

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