Add upgrading note for the "Dedicated StreamBucket class" RFC

[skip-ci]

Author: kocsismate

UPGRADING

@@ -251,6 +251,11 @@ PHP 8.4 UPGRADE NOTES
   . ReflectionClassConstant::__toString() and ReflectionProperty::__toString()
     now returns the attached doc comments.
 
+- Standard:
+  . stream_bucket_make_writeable() and stream_bucket_new() will now return a
+    StreamBucket instance instead of an stdClass instance.
+    RFC: https://wiki.php.net/rfc/dedicated_stream_bucket
+
 - SOAP:
   . Added support for clark notation for namespaces in class map.
     It is now possible to specify entries in a class map with clark notation