Mention removal in changelog

Author: japgolly

doc/changelog/3.0.0.md

@@ -35,6 +35,9 @@
   - Wrap router actions in `startTransition`, making the change of pages interruptible.
   - Allow creating custom `Route` instances outside the DSL model. A `Route` just wraps functions to turn a `Path` into a potential page and vice-versa.
 
+- The ability to globally turn React warnings into errors via sbt system properties has been removed.
+  It's test case had an error in it — it never worked as intended.
+
 ### Deprecated
 
 - Anything deprecated by React 18 is now deprecated in Scala