minor #1983 [Site] Leverage PHP-CS-Fixer parralel runner (smnandre)

This PR was squashed before being merged into the 2.x branch.

Discussion
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[Site] Leverage PHP-CS-Fixer parralel runner

(From symfony/symfony  #57687 😅 )

Commits
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c8bb8a9 [Site] Leverage PHP-CS-Fixer parralel runner

Author: javiereguiluz

ux.symfony.com/.php-cs-fixer.dist.php

@@ -23,6 +23,8 @@
 EOF;
 
 return (new PhpCsFixer\Config())
+    // @see https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/pull/7777
+    ->setParallelConfig(PhpCsFixer\Runner\Parallel\ParallelConfigFactory::detect())
     ->setRules([
         '@PHPUnit75Migration:risky' => true,
         '@Symfony' => true,