fix: restore nunit.framework reference in ThunderKit.Core.Tests.asmdef - #128

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, actions/upload-artifact@v4, dorny/test-reporter@v2, game-ci/unity-test-runner@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/