Install latest RubyGems when running make setup

What does this MR do and why?

This MR makes our local installations match what we do in our Docker images. For example: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/869cab67d704013bcbad1dd630b6efe56398c855/dockerfiles/gitlab-docs-lint-html.Dockerfile#L31.

Ruby comes with RubyGems, but it's never the latest version and can be quite old. So as part of a make setup, we can do: https://rubygems.org/pages/download. This has the benefit of keeping Bundler (https://bundler.io) up to date also. For example: https://blog.rubygems.org/2022/11/17/3.3.26-released.html.

CC @sarahgerman

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