add Ruby 3.0 to CI

[taichi-ishitani]

This PR is to add Ruby 3.0 to CI.

[clarkedb]

I see this adds Ruby 3.0 to CI, but will rubyzip 3.0.0 be the first release that supports ruby 3.0? Or does 2.3.2 already support it?

[hainesr]

That's a good question. Our CI on the 2.x branch died with Travis. I'll see about getting GitHub Actions backported and see where we are with that...

Thanks for raising this.

[petergoldstein]

@hainesr At this point we probably also want to add Ruby 3.1.

[hainesr]

Actions backported and confirmed that rubyzip 2.3.2 works with ruby 3.0 and 3.1. Note added to the README.