Update the LocalStack sandbox steps in .github/workflows/main.yml - #15

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, actions/setup-node@v4, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

lstk-project: PR review verdict YELLOW; not gating this job — https://v2.app.localstack.cloud/ci-runs/proj_b1e4056f/40161777411