File/Finder: Fix return type of key iterator method

See: https://mantis.ilias.de/view.php?id=41967

src/Filesystem/Finder/Iterator/RecursiveDirectoryIterator.php

@@ -43,7 +43,7 @@ public function __construct(private Filesystem $filesystem, protected string $di
     /**
      * @inheritdoc
      */
-    public function key(): string
+    public function key(): int|string
     {
         return key($this->files);
     }