Remove Deprecated for PHP 8.1

Add PHP Attribute `ReturnTypeWillChange` on ErrorContainer::next and ErrorContainer::rewind to temporarily suppress the deprecation notices: 
- `Deprecated: Return type of Opis\JsonSchema\Errors\ErrorContainer::next(): ?Opis\JsonSchema\Errors\ValidationError should either be compatible with Iterator::next(): void`
- `Deprecated: Return type of Opis\JsonSchema\Errors\ErrorContainer::rewind(): ?Opis\JsonSchema\Errors\ValidationError should either be compatible with Iterator::rewind(): void`.

Author: niconoe-

src/Errors/ErrorContainer.php

@@ -115,6 +115,7 @@ public function current(): ?ValidationError
     /**
      * @inheritDoc
      */
+    #[\ReturnTypeWillChange]
     public function next(): ?ValidationError
     {
         return next($this->errors) ?: null;
@@ -139,8 +140,9 @@ public function valid(): bool
     /**
      * @inheritDoc
      */
+    #[\ReturnTypeWillChange]
     public function rewind(): ?ValidationError
     {
         return reset($this->errors) ?: null;
     }
-}
+}