Run CI on ubuntu-20.04

[sato11]

Hi 👋 I came across and saw the CI not passing for some reason. This is the error message:

Requested Erlang/OTP version (21.3) not found in version list (should you be using option 'version-type': 'strict'?)

It seems this is because the ubuntu-latest is resolved ubuntu-22.04, which is not compatible with otp 21.3. Ref: https://github.com/erlef/setup-beam#compatibility-between-operating-system-and-erlangotp.

Downgrading os version might sound like a degradation, but I think we can excuse it since this is the way projects like plug run CI against older versions of otp.

This also updates actions/checkout from v2 to v3, consequently resolving deprecation warning that prompts using Nodejs v16. Ref: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

I validated that this fixes it after running the workflow in my fork, if you want to take a look 🙂

[Nitrino]

@sato11 Nice work! I just didn't have time to fix it ❤️

[sato11]

Thanks 😉