[skip ci] Add FFI BC to UPGRADING

Follow-up on commit 851e462

Author: Girgias

UPGRADING

@@ -27,6 +27,10 @@ PHP 8.3 UPGRADE NOTES
   . Class constants can now be accessed dynamically using the C::{$name} syntax.
     RFC: https://wiki.php.net/rfc/dynamic_class_constant_fetch
 
+- FFI:
+  . C functions that have a return type of void now return null instead of
+    returning the following object object(FFI\CData:void) { }
+
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 2. New Features
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