Reflection::getModifierNames() with readonly modifier

### Description

The following code:

```php
<?php

var_dump(Reflection::getModifierNames(ReflectionProperty::IS_READONLY));
```

Resulted in this output:
```
array(0) {
}
```

But I expected this output instead:
```
array(1) {
  [0]=>
  string(8) "readonly"
}
```


### PHP Version

PHP 8.1

### Operating System

_No response_