Update the promisse that ActionController::TestCase will be extracted

We are past 5.1 and it was not extrated yet, so while we still have
plans they will not be realized on 6.0, so it is better to not set
expectations of which release will exclude it just yet.

[ci skip]

actionpack/lib/action_controller/test_case.rb

@@ -26,7 +26,7 @@ def new_controller_thread # :nodoc:
     end
   end
 
-  # ActionController::TestCase will be deprecated and moved to a gem in Rails 5.1.
+  # ActionController::TestCase will be deprecated and moved to a gem in the future.
   # Please use ActionDispatch::IntegrationTest going forward.
   class TestRequest < ActionDispatch::TestRequest #:nodoc:
     DEFAULT_ENV = ActionDispatch::TestRequest::DEFAULT_ENV.dup