Use ruby/setup-ruby to simplify CI

[eregon]

• ruby/setup-ruby supports more versions than actions/setup-ruby.
• ruby/setup-ruby is faster to install Rubies than RVM (and much easier in GitHub Actions).

cc @hsbt @ioquatix

[eregon]

This currently drops jruby-head, do we want to keep it? I can try to support it in https://github.com/ruby/setup-ruby.

[eregon]

ruby/setup-ruby also allows testing basically any version on any platform.
Should we test 2.3, 2.2, jruby, truffleruby, ruby-head on macOS and Windows too?

[hsbt]

Thanks! I definitely prefer this.

[ioquatix]

Nice work!