fix: php 8.2 dynamic property warning in MapFieldIter (#11485)

addresses #11243

Defines `$key_type` in `MapFieldIter`, so as not to trigger PHP 8.2 warning. Note that this will change the visibility of `$key_type` from public in previous versions to private in this version. This is a breaking change, but anyone using `$key_type` in this way is misusing the library, so I think it's acceptable.

Closes #11485

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/11485 from bshaffer:fix-php-82-dynamic-warning-in-MapFieldIter b45c34d2be
PiperOrigin-RevId: 501040413

php/src/Google/Protobuf/Internal/MapFieldIter.php

@@ -49,6 +49,11 @@ class MapFieldIter implements \Iterator
      */
     private $container;
 
+    /**
+     * @ignore
+     */
+    private $key_type;
+
     /**
      * Create iterator instance for MapField.
      *