[Cart] Updated decpreciated discounts property on line items

[ludoboludo]

PR Summary:

We're using the discounts property in our cart a couple times. This is being depreciated so this PR aims to update that property to the recommended one discount_allocations.

Why are these changes introduced?

Fixes #1856

What approach did you take?

Replaced the property being called. It's also an array so shouldn't require anything else.

Other considerations

Here are our dev docs mentioning the depreciation.

Visual impact on existing themes

Shouldn't have any

Testing steps/scenarios

• Add an item in the cart that will have a discount applied to it. The Louise slide sandal should do 👍

Demo links

• Editor

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility

[lougoncharenko]

This looks good on my end. I tested the following and didn't find any issues:

• Tested the PR in mobile, tablet and desktop
• Tested on safari and chrome browsers
• Searched through the code base to ensure that all of items.discounts were replaced with item.discount_allocations.
• Added an item with a discount into the shopping cart to ensure the discount was applying
• Added more than one item to the cart to ensure the total was accurate and discounts were still applying.
• Tested for accessibility by turning on the screen reader and viewing the cart page. The screen reader read of the discounted price.