Automate release workflow with GitHub Actions

[dentarg]

The experience described in #134 made me think about our release workflow process. I think most of it is tasks suitable for non-humans. Maybe we can give the new shiny thing a try?

https://github.com/features/actions
https://developer.github.com/actions/

Past issue about doing it with Travis CI: #24

[dentarg]

https://github.com/marketplace/actions/publish-to-rubygems
https://github.com/cadwallion/publish-rubygems-action

Not sure if we should use that action over creating our own, because it isn't (yet) clear to me what guarantees we have that the action can't be updated to do malicious stuff.

[dentarg]

If I understand correctly, if we would use GitHub Actions to generate the changelog with GitHub Changelog Generator, we shouldn't need our own API token from GitHub, as each repo has its own: https://developer.github.com/actions/creating-workflows/storing-secrets/#github-token-secret

[dentarg]

We would have to change the Rubygems MFA settings to "UI only" to be able to release unattended. https://guides.rubygems.org/setting-up-multifactor-authentication/

[dentarg]

We would have to change the Rubygems MFA settings to "UI only" to be able to release unattended.

I did that so we can close #139