Complete the .ruby-version standardization

[george-ma]

What are you trying to accomplish?

This repo is well maintained and already follows the standards we set in place. b164c0b added the .ruby-version file, a required_ruby_version is already set in the .gemspec, a Gemfile.lock is already built, etc. This PR cleans up the last remaining mentions of Ruby version outside of the .ruby-version file (last two bullet points of the list below).

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