Use Python 3.12 in CI and for the Lambda functions - #8

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-python@v4, aws-actions/setup-sam@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Failed to download SAM CLI: Unexpected HTTP response: 502