README.md: Mention Ruby 3.1+ (#652)

Rather than an outdated required Ruby version.

Author: olleolleolle

README.md

@@ -29,7 +29,7 @@ If you feel like taking on any of this start an issue and update us on your prog
 
 ## Installation
 
-Install/add to Gemfile in Ruby 2.6+
+Install/add to Gemfile in Ruby 3.1+
 
 ```ruby
 gem 'rack-mini-profiler'