Update sbt, scripted-plugin to 1.12.15 - #778

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, coursier/cache-action@v6, coursier/setup-action@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/