Update CI yaml and update CHANGELOG entry

.gitlab-ci.yml

-image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.3"
+image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.3-git-2.7-phantomjs-2.1"
 
 cache:
   key: "ruby-231"
@@ -140,7 +140,7 @@ spinach 9 10: *spinach-knapsack
 
 # Execute all testing suites against Ruby 2.1
 .ruby-21: &ruby-21
-  image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.1"
+  image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.1-git-2.7-phantomjs-2.1"
   <<: *use-db
   only:
     - master

CHANGELOG

@@ -14,6 +14,7 @@ v 8.13.0 (unreleased)
   - Use a ConnectionPool for Rails.cache on Sidekiq servers
   - Only update issuable labels if they have been changed
   - Take filters in account in issuable counters. !6496
+  - Use custom Ruby images to test builds (registry.gitlab.com/gitlab-org/gitlab-build-images:*)
   - Revoke button in Applications Settings underlines on hover.
   - Fix Long commit messages overflow viewport in file tree
   - Revert avoid touching file system on Build#artifacts?
@@ -140,7 +141,6 @@ v 8.12.0
   - Fix issue with slash commands not loading on new issue page
   - Fix inconsistent background color for filter input field (ClemMakesApps)
   - Remove prefixes from transition CSS property (ClemMakesApps)
-  - Use custom Ruby images to test builds (registry.gitlab.com/gitlab-org/gitlab-build-images:*)
   - Add Sentry logging to API calls
   - Add BroadcastMessage API
   - Use 'git update-ref' for safer web commits !6130