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”)

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

[george-ma]

I dropped Rails 6.0 support on CI tests to get CI passing as this version has had security support dropped for almost a year (~11 months)

Let me know if I should split up this removal commit and a1e3910 into their own PRs.

[jenshenny]

Closing in favour of: https://github.com/Shopify/packwerk/pull/407