Clarify SOAP changes in UPGRADING file

Update SOAP section to clarify changes in classmap option and WSDL parsing.

Author: LamentXU123

UPGRADING

@@ -157,7 +157,7 @@ PHP 8.6 UPGRADE NOTES
 - SOAP:
   . The "classmap" option of SoapClient and SoapServer now rejects arrays
     containing integer keys. Previously, sparse integer-keyed and mixed-keyed
-    arrays could be accepted. (Weilin Du)
+    arrays could be accepted.
   . WSDL/XML Schema parsing now rejects out-of-range integer values for
     occurrence constraints and integer restriction facets. Negative minOccurs
     and maxOccurs values are rejected as well.