Drop support for Ruby 2.1

Given that the 9.0 release of GitLab will mark the end of the support
for Ruby 2.1, we can stop testing against Ruby 2.1.

Fixes #27251

.gitlab-ci.yml

@@ -163,64 +163,7 @@ spinach 7 10: *spinach-knapsack
 spinach 8 10: *spinach-knapsack
 spinach 9 10: *spinach-knapsack
 
-# Execute all testing suites against Ruby 2.1
-.ruby-21: &ruby-21
-  image: "dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.1-git-2.7-phantomjs-2.1"
-  <<: *use-db
-  only:
-    - master@gitlab-org/gitlab-ce
-    - master@gitlab-org/gitlab-ee
-    - master@gitlab/gitlabhq
-    - master@gitlab/gitlab-ee
-  cache:
-    key: "ruby21"
-    paths:
-      - vendor/ruby
-
-.rspec-knapsack-ruby21: &rspec-knapsack-ruby21
-  <<: *rspec-knapsack
-  <<: *dedicated-runner
-  <<: *ruby-21
-
-.spinach-knapsack-ruby21: &spinach-knapsack-ruby21
-  <<: *spinach-knapsack
-  <<: *dedicated-runner
-  <<: *ruby-21
-
-rspec 0 20 ruby21: *rspec-knapsack-ruby21
-rspec 1 20 ruby21: *rspec-knapsack-ruby21
-rspec 2 20 ruby21: *rspec-knapsack-ruby21
-rspec 3 20 ruby21: *rspec-knapsack-ruby21
-rspec 4 20 ruby21: *rspec-knapsack-ruby21
-rspec 5 20 ruby21: *rspec-knapsack-ruby21
-rspec 6 20 ruby21: *rspec-knapsack-ruby21
-rspec 7 20 ruby21: *rspec-knapsack-ruby21
-rspec 8 20 ruby21: *rspec-knapsack-ruby21
-rspec 9 20 ruby21: *rspec-knapsack-ruby21
-rspec 10 20 ruby21: *rspec-knapsack-ruby21
-rspec 11 20 ruby21: *rspec-knapsack-ruby21
-rspec 12 20 ruby21: *rspec-knapsack-ruby21
-rspec 13 20 ruby21: *rspec-knapsack-ruby21
-rspec 14 20 ruby21: *rspec-knapsack-ruby21
-rspec 15 20 ruby21: *rspec-knapsack-ruby21
-rspec 16 20 ruby21: *rspec-knapsack-ruby21
-rspec 17 20 ruby21: *rspec-knapsack-ruby21
-rspec 18 20 ruby21: *rspec-knapsack-ruby21
-rspec 19 20 ruby21: *rspec-knapsack-ruby21
-
-spinach 0 10 ruby21: *spinach-knapsack-ruby21
-spinach 1 10 ruby21: *spinach-knapsack-ruby21
-spinach 2 10 ruby21: *spinach-knapsack-ruby21
-spinach 3 10 ruby21: *spinach-knapsack-ruby21
-spinach 4 10 ruby21: *spinach-knapsack-ruby21
-spinach 5 10 ruby21: *spinach-knapsack-ruby21
-spinach 6 10 ruby21: *spinach-knapsack-ruby21
-spinach 7 10 ruby21: *spinach-knapsack-ruby21
-spinach 8 10 ruby21: *spinach-knapsack-ruby21
-spinach 9 10 ruby21: *spinach-knapsack-ruby21
-
 # Other generic tests
-
 .ruby-static-analysis: &ruby-static-analysis
   variables:
     SIMPLECOV: "false"