[php-src] Issue #20606: #[\NoDiscard] should affect inheritance and interface implementation

Issue: https://github.com/php/php-src/issues/20606
Author: Sharom

### Description

`#[\NoDiscard]` attribute added in v8.5 is brilliant, but there is an unexpected nuance.

Using an attribute in an interface method does not affect the implementing class.
The same is true for a child class that overrides the parent method.

In my opinion, this behavior is contrary to common sense.

**For example:**

```php
interface Equatable
{
    #[\NoDiscard]
    public function equals(object $other): bool;
}

final readonly class IntValue implements Equatable
{
    public function __construct(private int $value) {}

    #[\Override]
    public function equals(object $other): bool
    {
        return $other instance of static && $this->value == $other->value;
    }
}

$x = new IntValue(10);
$y = new IntValue(20);

$x->equals($y); // Warning is naturally expected here,
                // but [\NoDiscard] attribute is not "inherited"
                // in case of the interface implementation.
```

I may have missed something very important here, let's discuss it.