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
```

SpecBaseObject.php、SpecObjectInterface.php、spec/Paths.php、spec/PathItem.php、spec/Responses.phpのresolveReferencesシグネチャに加え、spec/Reference.phpのコンストラクタとresolveメソッドを確認してください。警告で特定された暗黙的にnullableなパラメータを更新し、PHP 8.4でこれらの非推奨警告が表示されなくなったことを検証してください。

索引モデルが issue の本文から書いたものです。