Fix deprecation notices for legacy order item keys.

[mikejolley]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

I've split this off from https://github.com/woocommerce/woocommerce/pull/44181/files and this fixes the same issue as #41567

Under PHP 8.2 we see some notices on the frontend:

PHP Deprecated: Creation of dynamic property WC_Order_Item_Product::$legacy_values is deprecated
PHP Deprecated: Creation of dynamic property WC_Order_Item_Product::$legacy_cart_item_key is deprecated
PHP Deprecated: Creation of dynamic property WC_Order_Item_Shipping::$legacy_package_key is deprecated

By declaring the properties we fix the issue.

Closes #41567

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

Before using this PR:

1. Install query monitor plugin on a store running PHP 8.2+
2. browse to the checkout page with items in your cart logged in as admin
3. you'll see "Creation of dynamic property WC_Order_Item_Shipping::$legacy_package_key is deprecated" notices caught by query monitor

After applying this PR:

1. Repeat the above steps. No notices will be caught by query monitor.

Changelog entry

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Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

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Comment

[github-actions]

Hi @opr,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[github-actions]

Test Results Summary

Commit SHA: 68ee51a

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 38s
E2E Tests	308	0	0	6	0	314	6m 38s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[opr]

Thanks Mike, this is working fine for me. Just take a look at CI because it is stuck and also the testing instructions are fine for devs, but when this will be tested by GlobalStep they won't be able to follow the instructions (e.g. "apply this patch") so can we re-write them to be simpler (or somehow mark this as not needing testing)?