Update requirements to indicate Ruby 2.7

doc/install/requirements.md

@@ -45,7 +45,13 @@ Please consider using a virtual machine to run GitLab.
 
 ### Ruby versions
 
-GitLab requires Ruby (MRI) 2.6. Beginning in GitLab 12.2, we no longer support Ruby 2.5 and lower.
+From GitLab 13.6:
+
+- Ruby 2.7 and later is required.
+
+From GitLab 12.2:
+
+- Ruby 2.6 and later is required.
 
 You must use the standard MRI implementation of Ruby.
 We love [JRuby](https://www.jruby.org/) and [Rubinius](https://github.com/rubinius/rubinius#the-rubinius-language-platform), but GitLab