Add return type declaration for current() method in BatchIterator (#550)

Author: wiewiurdp

src/contracts/Repository/Iterator/BatchIterator.php

@@ -36,7 +36,7 @@ public function __construct(
     }
 
     #[\ReturnTypeWillChange]
-    public function current()
+    public function current(): mixed
     {
         if (!$this->isInitialized()) {
             $this->initialize();