Fix railties/CHANGELOG.md for cookie_serializer

The framework defaults file sets the serializer to `:hybrid` instead of
`:marshal`, but we can just remove the actual setting and referer to the
defaults file instead. [ci-skip]

Author: p8

railties/CHANGELOG.md

@@ -10,7 +10,7 @@
     or `config/initializers/cookies_serializer.rb`
 
     The default value for `cookies_serializer` (`:json`) has been moved to `config.load_defaults("7.0")`.
-    The new framework defaults file sets the serializer to `:marshal`.
+    The new framework defaults file can be used to upgrade the serializer.
 
     *Alex Ghiculescu*