Avoid deprecated SplObjectStorage::attach (#224)

`SplObjectStorage::attach()` was deprecated with PHP 8.5 [1].
Its usage is replaced by `SplObjectStorage::offsetSet()`,
which provides the same functionality.

[1]: php/php-src#19424

Author: eliashaeussler

src/Worker/ContextWorkerPool.php

@@ -256,7 +256,7 @@ private function pull(): Worker
                             throw new WorkerException('Worker factory did not create a viable worker');
                         }
 
-                        $workers->attach($worker, 0);
+                        $workers->offsetSet($worker, 0);
                         return $worker;
                     });