Handle deprecation of Rack::File in Rack 3.1

When using Rack >= 3.0.0 you get this error message on boot an app with Flipper UI :

```
warning: Rack::File is deprecated and will be removed in Rack 3.1
```

This PR detects if `Rack::Files` is defined and tries to use it instead.

Source : rack/rack#1720

lib/flipper/ui/actions/file.rb

@@ -8,7 +8,8 @@ class File < UI::Action
         route %r{(images|css|js)/.*\Z}
 
         def get
-          Rack::File.new(public_path).call(request.env)
+          klass = Rack.release >= "2.1" ? Rack::Files : Rack::File
+          klass.new(public_path).call(request.env)
         end
       end
     end