Add rector for _cake_core_ deprecation.

config/rector/cakephp51.php

@@ -0,0 +1,9 @@
+<?php
+declare(strict_types=1);
+
+use Cake\Upgrade\Rector\Set\CakePHPSetList;
+use Rector\Config\RectorConfig;
+
+return static function (RectorConfig $rectorConfig): void {
+    $rectorConfig->sets([CakePHPSetList::CAKEPHP_51]);
+};

config/rector/sets/cakephp51.php

@@ -0,0 +1,14 @@
+<?php
+declare(strict_types=1);
+
+use Rector\Config\RectorConfig;
+use Rector\Renaming\Rector\String_\RenameStringRector;
+
+# @see https://book.cakephp.org/5/en/appendices/5-1-migration-guide.html
+return static function (RectorConfig $rectorConfig): void {
+
+    $rectorConfig->ruleWithConfiguration(RenameStringRector::class, [
+        // Rename the cache configuration used by translations.
+        '_cake_core_' => '_cake_translations_',
+    ]);
+};

src/Rector/Set/CakePHPSetList.php

@@ -72,6 +72,11 @@ final class CakePHPSetList implements SetListInterface
      */
     public const CAKEPHP_50 = __DIR__ . '/../../../config/rector/sets/cakephp50.php';
 
+    /**
+     * @var string
+     */
+    public const CAKEPHP_51 = __DIR__ . '/../../../config/rector/sets/cakephp51.php';
+
     /**
      * @var string
      */