Centralize Ruby Version to `.ruby-version`

[alexcrocha]

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 present in the dev.yml Ruby task (before: - ruby: x.x.x, after: - ruby)
• [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
• [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”)

This PR will be merged if there isn't any activity after 4 weeks.

[alexcrocha]

CI is failing on rails-edge because it no longer supports Ruby 3.0 (source). With Ruby 3.0 reaching its EOL at the end of this month (source) I suggest we also drop our support for it.

[kmcphillips]

I'm good with removing it. We're going to do a release anyway, so may as well bump the minor version and just say it's 3.1.0+.

Will need to update the CI matrix: https://github.com/Shopify/measured-rails/blob/d9d4a7ecdb22b045d043f6df277478e34ee0c8ed/.github/workflows/ci.yml#L14

And all these changes mirrored to measured.

[paracycle]

CI is failing on rails-edge because it no longer supports Ruby 3.0 (source).

I already fixed this problem on main. Rebasing will fix it for you too.