Bump required Ruby version in install doc

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57350

doc/install/installation.md

@@ -139,8 +139,8 @@ Then select 'Internet Site' and press enter to confirm the hostname.
 
 The Ruby interpreter is required to run GitLab.
 
-**Note:** The current supported Ruby (MRI) version is 2.3.x. GitLab 9.0 dropped
-support for Ruby 2.1.x.
+**Note:** The current supported Ruby (MRI) version is 2.5.x. GitLab 11.6
+  dropped support for Ruby 2.4.x.
 
 The use of Ruby version managers such as [RVM], [rbenv] or [chruby] with GitLab
 in production, frequently leads to hard to diagnose problems. For example,