Remove warning about setup-ruby and continue to maintain

[AlexWayfer]

I see the warning:

DO NOT USE THIS ACTION

ruby/setup-ruby provides everything that this action has provided.

But it's not true.

With rbenv I can switch versions on-the-flight, for example gem generator supports only Ruby >= 3, but the gem template supports >= 2.6.

So, how can I do this with ruby/setup-ruby? Nohow, I think.

But with matrix like 2.6, 2.7, ... I can use the base Ruby versions for the template testing, and rbenv install 3.0 && rbenv local 3.0 for generation in pre-steps.