Update Ruby version in official CI templates

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

changelogs/unreleased/update-ruby-version-on-official-ci-templates.yml

+---
+title: Update Ruby version in official CI templates
+merge_request: 23585
+author: Takuya Noguchi
+type: other

lib/gitlab/ci/templates/Pages/Jekyll.gitlab-ci.yml

 # Template project: https://gitlab.com/pages/jekyll
 # Docs: https://docs.gitlab.com/ce/pages/
-image: ruby:2.3
+image: ruby:2.6
 
 variables:
   JEKYLL_ENV: production
   LC_ALL: C.UTF-8
 
 before_script:
+  - gem install bundler
   - bundle install
 
 test:

lib/gitlab/ci/templates/Pages/Middleman.gitlab-ci.yml

 # Full project: https://gitlab.com/pages/middleman
-image: ruby:2.3
+image: ruby:2.6
 
 cache:
   paths:

lib/gitlab/ci/templates/Pages/Nanoc.gitlab-ci.yml

 # Full project: https://gitlab.com/pages/nanoc
-image: ruby:2.3
+image: ruby:2.6
 
 pages:
   script:

lib/gitlab/ci/templates/Pages/Octopress.gitlab-ci.yml

 # Full project: https://gitlab.com/pages/octopress
-image: ruby:2.3
+image: ruby:2.6
 
 pages:
   script: