fix(ci): remove disallowed Slack notification - #508

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, actions/setup-go@v5, actions/upload-artifact@v4, codecov/codecov-action@v3.1.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/