ACP2E-1001: Customer Zip Code input validation rule not working

- Updated code as per CR comments.

Author: Mohan Ahuja

app/code/Magento/Customer/Model/Metadata/Form/Postcode.php

@@ -22,11 +22,6 @@ class Postcode extends Text
      */
     protected DirectoryHelper $directoryHelper;
 
-    /**
-     * @var StringUtils
-     */
-    protected $_string;
-
     /**
      * @param MagentoTimezone $localeDate
      * @param PsrLogger $logger
@@ -50,10 +45,7 @@ public function __construct(
         StringUtils $stringHelper = null
     ) {
         $this->directoryHelper = $directoryHelper;
-        if ($stringHelper === null) {
-            $stringHelper = \Magento\Framework\App\ObjectManager::getInstance()->get(StringUtils::class);
-        }
-        $this->_string = $stringHelper;
+        $stringHelper = $stringHelper ?? \Magento\Framework\App\ObjectManager::getInstance()->get(StringUtils::class);
         parent::__construct(
             $localeDate,
             $logger,

app/code/Magento/Customer/Test/Unit/Model/Metadata/Form/PostcodeTest.php

@@ -17,7 +17,7 @@
 class PostcodeTest extends AbstractFormTestCase
 {
     /** @var StringUtils */
-    protected StringUtils $stringHelper;
+    private StringUtils $stringHelper;
 
     /**
      * @var DirectoryHelper|MockObject