UPGRADING: (real) cast has been removed

.. but the removal wasn't mentioned in the `UPGRADING` docs yet.

Ref:
* #5220
* c9db322

Closes GH-6440.

[ci-skip]

UPGRADING

@@ -29,6 +29,7 @@ PHP 8.0 UPGRADE NOTES
   . Removed ability to call non-static methods statically.
     Thus `is_callable` will fail when checking for a non-static method with a
     classname (must check with an object instance).
+  . Removed (real) cast.
   . Removed (unset) cast.
   . Removed track_errors ini directive. This means that $php_errormsg is no
     longer available. The error_get_last() function may be used instead.