PHP 8.4.x deprecation warnings

With the release of PHP 8.4, implicitly marking parameters as nullable is now deprecated.

The dev-master version of this package contains a few warnings because of this:

```
DEPRECATED cebe\openapi\SpecBaseObject::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/SpecBaseObject.php on line 396.
DEPRECATED cebe\openapi\SpecObjectInterface::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/SpecObjectInterface.php on line 43.
DEPRECATED cebe\openapi\spec\Paths::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Paths.php on line 249.
DEPRECATED cebe\openapi\spec\PathItem::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/PathItem.php on line 153.
DEPRECATED cebe\openapi\spec\Responses::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Responses.php on line 239.
DEPRECATED cebe\openapi\spec\Reference::__construct(): Implicitly marking parameter $to as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 72.
DEPRECATED cebe\openapi\spec\Reference::resolve(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 179.
DEPRECATED cebe\openapi\spec\Reference::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead in vendor/cebe/php-openapi/src/spec/Reference.php on line 366.
```

Example:

```php
public function resolveReferences(ReferenceContext $context = null); // SpecObjectInterface.php #43
```