Centralize Ruby Version to `.ruby-version`

[jenshenny]

What are you trying to accomplish?

The .ruby-version file is the ecosystem standard for defining a Ruby version. This PR adds the .ruby-version file, ensures a required_ruby_version is set, and removes all other references to Ruby in this repository, aligning it with the standard.

What should reviewers focus on?

[!IMPORTANT] Please verify the following before merging:

Verify that the changes in the PR meets the following requirements or adjust manually to make it compliant:

• [x] .ruby-version file is present with the correct Ruby version defined
• [x] A required_ruby_version in your gemspec is set
• [x] There is no Ruby version/requirement referenced in the Gemfile (no lines with ruby <some-version>)
• [x] A Gemfile.lock is built with the defined Ruby version
• [x] The version of Rubocop installed is 1.61.0 or greater
• [x] There is no TargetRubyVersion defined in rubocop.yml (reads from required_ruby_version on Rubocop 1.61.0)
• [x] There is no Ruby argument present in ruby/setup-ruby Github Actions that do not run on a Ruby matrix (no lines with ruby-version: “x.x”)

[jenshenny]

Hmm CI is failing on pull_request but not on push... going need to take a closer look at this.

[jenshenny]

I think it's from updating bundler https://github.com/Shopify/bootboot/pull/64#issuecomment-1805554376