fix: Use thumb for next-up/continue instead of posters - #1051

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4.1.1, ValentinVignal/action-dart-analyze@v2.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/