Add UPGRADING note about SNMP class constant type declarations

[skip-ci]

Author: kocsismate

UPGRADING

@@ -77,6 +77,9 @@ PHP 8.3 UPGRADE NOTES
   . The file() flags error check now catches all invalid flags. Notably
     FILE_APPEND was previously silently accepted.
 
+- SNMP:
+  . The type of SNMP class constants are now declared.
+
 ========================================
 2. New Features
 ========================================