[5.8] HasAttributes::originalIsEquivalent to public

[pablorsk]

HasAttributes::originalIsEquivalent function is great for observers... For example, currently, on saving() event:

    public function saving($product) {
        if ($product->stock !== $product->getOriginal('stock')) {
            // do stuff
        }
    }

After:

    public function saving($product) {
        if (!$product->originalIsEquivalent('stock')) {
            // do stuff
        }
    }

Its possible to change this? Or is there a reason to keep it that way?

Closes #26383, Related with PR #26384