Merge branch 'jl-update-ruby-2-5-docs' into 'master'

Update minimum ruby version to 2.5.

See merge request gitlab-org/gitlab-ce!25496

(cherry picked from commit 74cf92aa)

ac34b4ac Update minimum ruby version to 2.5.
a538b6db Update example versions

doc/install/requirements.md

@@ -33,7 +33,7 @@ Please consider using a virtual machine to run GitLab.
 ## Ruby versions
-GitLab requires Ruby (MRI) 2.3. Support for Ruby versions below 2.3 (2.1, 2.2) will stop with GitLab 8.13.
+GitLab requires Ruby (MRI) 2.5. Support for Ruby versions below 2.5 (2.3, 2.4) will stop with GitLab 11.6.
 You will have to use the standard MRI implementation of Ruby.
 We love [JRuby](https://www.jruby.org/) and [Rubinius](https://rubinius.com) but GitLab