ci: Test with Ruby 2.6, 2.7, 3.0, 3.1 and head

[kou]

We need to use ruby/setup-ruby instead of actions/setup-ruby. actions/setup-ruby is deprecated. It doesn't support newer Rubies.

See: https://github.com/actions/setup-ruby/commit/e932e7af67fc4a8fc77bd86b744acd4e42fe3543

We can use bundler-cache to run Bundler and cache the result with ruby/setup-ruby: https://github.com/ruby/setup-ruby#bundler