Add notes about dropping support for Ruby 2.1.x

[ci skip]

doc/install/installation.md

@@ -109,7 +109,8 @@ Then select 'Internet Site' and press enter to confirm the hostname.
 
 ## 2. Ruby
 
-**Note:** The current supported Ruby versions are 2.1.x and 2.3.x. 2.3.x is preferred, and support for 2.1.x will be dropped in the future.
+**Note:** The current supported Ruby version is 2.3.x. GitLab 9.0 dropped support
+for Ruby 2.1.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,

doc/update/8.17-to-9.0.md

@@ -25,9 +25,8 @@ sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
 
 ### 3. Update Ruby
 
-We will continue supporting Ruby < 2.3 for the time being but we recommend you
-upgrade to Ruby 2.3 if you're running a source installation, as this is the same
-version that ships with our Omnibus package.
+NOTE: GitLab 9.0 only supports Ruby 2.3.x and dropped support for Ruby 2.1.x. Be
+sure to upgrade your interpreter if necessary.
 
 You can check which version you are running with `ruby -v`.