Use cimg/ruby instead of deprecated circleci/ruby images

Author: koic

.circleci/config.yml

@@ -37,16 +37,16 @@ workflows:
       - documentation-checks
       - rake_default:
           name: Ruby 2.5
-          image: circleci/ruby:2.5
+          image: cimg/ruby:2.5
       - rake_default:
           name: Ruby 2.6
-          image: circleci/ruby:2.6
+          image: cimg/ruby:2.6
       - rake_default:
           name: Ruby 2.7
-          image: circleci/ruby:2.7
+          image: cimg/ruby:2.7
       - rake_default:
           name: Ruby 3.0
-          image: circleci/ruby:3.0
+          image: cimg/ruby:3.0
       - rake_default:
           name: Ruby HEAD
           image: rubocophq/circleci-ruby-snapshot:latest # Nightly snapshot build