Drop deprecated `taint` mechanism; add Ruby 3 build to CI

Taint checking has been deprecated starting Ruby 2.7: https://bugs.ruby-lang.org/issues/16131. It is well described, for example, here: https://blog.saeloun.com/2020/02/18/ruby-2-7-access-and-setting-of-safe-warned-will-become-global-variable.html

As a result, we receive warnings in our CI when running Ruby 3 builds, e.g.: https://gitlab.com/gitlab-org/gitlab/-/jobs/1524901589#L409

Related to gitlab-org/gitlab#339001 (closed)