CI: Use bundler-cache, and rely on on ruby/setup-ruby to have patch versions of Ruby

[olleolleolle]

This PR uses the bundler-cache feature of the Ruby-maintained Action ruby/setup-ruby.

See ruby/setup-ruby parameters declaration for more information.

[olleolleolle]

About quoting "3.0":

This is a very small change, but which avoids a Float-to-String loss of characters.

An example (not from this project) of what it can look like before this change, in this picture:

Read more details, if you like: https://github.com/actions/runner/issues/849

[olleolleolle]

If this PR is merged, it also does what #1466 did (in another way).

[vsppedro]

Thank you for working on this!

I tried to use the ruby/setup-ruby, but it did not work as you can see in this PR: https://github.com/thoughtbot/shoulda-matchers/pull/1397.

If you like to understand why the CI is failing take a look at this comment: https://github.com/thoughtbot/shoulda-matchers/pull/1397#issuecomment-765059033

I intended to test these options: https://github.com/thoughtbot/shoulda-matchers/pull/1397#issuecomment-782710793, but I forgot. 😓