[skip ci] Add docs for json_validate in UPGRADING*

UPGRADING

@@ -47,6 +47,10 @@ PHP 8.3 UPGRADE NOTES
 6. New Functions
 ========================================
 
+- JSON:
+  . Added json_validate(), which returns whether the json is valid for
+    the given $depth and $options.
+
 - Posix:
   . Added posix_sysconf call to get runtime informations.
 

UPGRADING.INTERNALS

@@ -25,6 +25,9 @@ PHP 8.3 INTERNALS UPGRADE NOTES
 3. Module changes
 ========================
 
+ a. ext/json
+   - A new function php_json_validate_ex has been added to check if the
+     provided C string is valid for the given depth and options.
 ========================
 4. OpCode changes
 ========================