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:

• [ ] .ruby-version file is present with the correct Ruby version defined
• [ ] A required_ruby_version in your gemspec is set
• [ ] There is no Ruby version present in the dev.yml Ruby task (before: - ruby: x.x.x, after: - ruby)
• [ ] There is no Ruby version/requirement referenced in the Gemfile (no lines with ruby <some-version>)
• [ ] A Gemfile.lock is built with the defined Ruby version
• [ ] The version of Rubocop installed is 1.61.0 or greater
• [ ] There is no TargetRubyVersion defined in rubocop.yml (reads from required_ruby_version on Rubocop 1.61.0)
• [ ] 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”)

To establish consistency, the required_ruby_version is set to 3.0. If you think that another version is a better fit, please make the applicable changes.

Please merge this PR if it looks good, this PR will be addressed if there isn't any activity after 4 weeks.

[casperisfine]

There is no TargetRubyVersion defined in rubocop.yml

This should not apply to open source nor to libraries in general. Contrary to apps, they support multiple Ruby versions.

[jenshenny]

In RuboCop 1.61.0, this value will be read from required_ruby_version and will take the oldest supported version from that spec. https://github.com/rubocop/rubocop/pull/12645. This will remove having to update the value when you want to update the minimum supported version.

If you do not want to update Rubocop for this feature due to the other changes it causes, I'll remove the changes related to RuboCop.

[casperisfine]

If you do not want to update Rubocop for this feature due to the other changes it causes, I'll remove the changes related to RuboCop.

Yeah it's fine, let's keep a minor duplication for now. I'll update Rubocop when it makes sense.

[jenshenny]

Sweet! I removed the Rubocop bump ✅