Add GMP entry in UPGRADING

Co-authored-by: Gina Peter Banyard <girgias@php.net>

Author: LamentXU123

UPGRADING

@@ -174,7 +174,7 @@ PHP 8.6 UPGRADE NOTES
 4. Deprecated Functionality
 ========================================
 
-- New warnings and exceptions:
+- GMP
   . The shift (<<, >>) and exponentiation (**) operators on GMP objects now
     emit a deprecation warning when converting a float right operand to int
     loses precision.