bump checkout, setup-python, build-push-action to v7; cache to v6; login-action, setup-buildx-action to v4; setup-oras to v2 - #4

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: ilammy/msvc-dev-cmd@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/