Update GitHub actions workflow

[inooid]

Description

• Updates the matrix table to more modern versions
• Updates the setup elixir action to a maintained version by Erlang (see: https://github.com/actions/setup-elixir#setup-elixir)
• Added dependency caching as it can speed up CI

For now I left mix credo out of it as it was not in the current pipeline and there's probably a lot to fix. Let's do this in future PRs. There's a lot of deprecated methods across versions and there's probably a discussion to be had over which OTP + Elixir versions we should support.

Note: This CI pipeline is inspired by the setup of: https://github.com/swoosh/swoosh

[inooid]

If someone has the chance to kick off the CI for this PR, I'd be very grateful 🙇 (or give me permission to kick it off myself as otherwise I'll have to continuously bother the maintainers on every push 😅)

[inooid]

For now I've made it do the same things as the current CI does. We can do follow up PRs cleaning up (e.g. deprecated methods, etc.) and adding credo and/or dialyxir. If necessary I can rebase and clean up the commits or we squash and merge depending on what's preferred (as there's no contribution document).

Feel free to review where you see fit 👍

[BlakeWilliams]

Thanks for fixing this up! ❤️