Update $_required typehint

[DjordyKoert]

NelmioApiDocBundle is currently attempting to upgrade PHPStan to level 6.

Our annotations which extend AbstractAnnotation are generating errors. A simple typehint fixes this which is the reason for this PR :)

https://github.com/nelmio/NelmioApiDocBundle/blob/a585e502725d7e3cfcec25f7a3d59443842006da/src/Annotation/Model.php#L30

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  Line   Annotation/Model.php                                                                                                
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  30     Property Nelmio\ApiDocBundle\Annotation\Model::$_required type has no value type specified in iterable type array.  
         💡 See: https://phpstan.org/blog/solving-phpstan-no-value-type-specified-in-iterable-type                           
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