[TASK] Use the PHP version from composer.json for Rector

https://getrector.com/documentation/set-lists#content-php-sets

Author: oliverklee

Build/rector/config.php

@@ -13,6 +13,7 @@
             __DIR__ . '/../../tests',
         ]
     )
+    ->withPhpSets()
     ->withComposerBased()
     ->withSets([
         // SetList::CODE_QUALITY,