Do not allow to redeem gift cards when purchasing other gift cards

[AndreiMaz]

nopCommerce version: 4.70.4

Do not allow to redeem gift cards when purchasing other gift cards. So when a customer will try to apply a gift card, we should validate whether it could be used or not. We already have the same logic applied to discounts (do not allow to purchase gift cards) - please check the "ValidateDiscountAsync" method of the "DiscountService":

if (await _productService.HasAnyGiftCardProductAsync(cartProductIds))
{...}

Let's implement the same logic in "GiftCardService.IsGiftCardValidAsync" and "ShoppingCartController.ApplyGiftCard" methods

[AndreiMaz]

original issue moved to #6681 but let's implement this one as well