Decouple django and python versions from other dependencies

[MarcoGlauser]

The main change is decoupling django and python versions from other libraries. That way, when a new version of django comes out, we can add it without having to update the "latest", "previous" and "legacy" dependencies. Same thought for new python versions. If a new version comes out or we deprecate an old version, we can add it without having to update other things.

At the same time, this PR updates the github actions versions to their latest version to avoid deprecation notices.