fix: Use valid PHP-CS-Fixer migration ruleset

Author: qodo-code-review[bot]

bbbeasy-backend/.php-cs-fixer.php

@@ -50,7 +50,7 @@
 $config
     ->setRiskyAllowed(true)
     ->setRules([
-        '@PHP8x5Migration' => true,
+        '@PHP84Migration' => true,
         '@PhpCsFixer' => true,
         '@PhpCsFixer:risky' => true,
         'general_phpdoc_annotation_remove' => ['annotations' => ['expectedDeprecation']], // one should use PHPUnit built-in method instead