ci: switch to the ruby/setup-ruby action

[parndt]

What problem is this PR intended to solve?

This switches the setup-ruby action to the one created by the
ruby organisation which has more
advanced features than the one already in use.

Most notably, this handles installing Bundler, bundling, and caching the
resulting bundle.

Have you included adequate test coverage?

Yes, which is to say it hasn't changed any code or tests, just how they
are run.

Does this change affect the behavior of either the C or the Java implementations?

No, just how CI runs, and you can see the results on this PR 😄.

[flavorjones]

@parndt Sweet! Thanks very much for showing us how to do this. Merging!

[parndt]

@flavorjones you're very welcome; thanks for all the work on Nokogiri! Happy to have helped. 🎉