Use cimg/ruby instead of deprecated circleci/ruby image

[koic]

Follow up f2b1319f.

circleci/ruby image is also deprecated. And cimg/ruby image cannot be used, cimg/ruby:3.1 is used instead.

Starting container cimg/ruby
Warning: No authentication provided, using CircleCI credentials for pulls from Docker Hub.
  image cache not found on this host, downloading cimg/ruby

Error response from daemon: manifest for cimg/ruby:latest not found: manifest unknown: manifest unknown

https://app.circleci.com/pipelines/github/rubocop/rubocop-performance/789/workflows/e2da53d9-cd49-46a0-aacf-040bb50606b1/jobs/5240

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.