Bump @angular/core from 20.3.19 to 20.3.28 in /frontend - #538

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, gh-bot/fix-labeler@master. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/