Use cimg/ruby instead of deprecated circleci/ruby image

Follow up rubocop/rubocop-performance#301

Author: koic

.circleci/config.yml

@@ -6,7 +6,7 @@ jobs:
       image:
         description: "Name of the Docker image."
         type: string
-        default: "circleci/ruby"
+        default: "cimg/ruby:3.1"
     docker:
       - image: << parameters.image >>
     environment:
@@ -23,7 +23,7 @@ jobs:
   # Miscellaneous tasks
   documentation-checks:
     docker:
-      - image: circleci/ruby
+      - image: cimg/ruby:3.1
     steps:
       - checkout
       - run: bundle install