Remove restrictions on rubocop ruby version

[igas]

We are on ruby 3.1 .ruby-version.

Rubocop supports .ruby-version lookup, so we don't need to define it and keep it up to date.

https://docs.rubocop.org/rubocop/1.56/configuration.html#setting-the-target-ruby-version

It'll enable better cop support for newer ruby functionality.