fix: remove fixed engines field and add npm install cooldown - #23

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: amannn/action-semantic-pull-request@0723387faaf9b38adef4775cd42cfd5155ed6017. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/