Fix deprecated messages on PHP 8.2 from using FILTER_SANITIZE_STRING

[mattallan]

Changes proposed in this Pull Request:

While testing another PR (#2837) I noticed a bunch of deprecated warnings in my error logs while trying to purchase a product using Google Pay and Apple Pay:

[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1175
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1176
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1177
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1178
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1179
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1180
[22-May-2024 05:10:49 UTC] PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in /Users/matt/local/woo/wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-payment-request.php on line 1183

Looks like the FILTER_SANITIZE_STRING constant has been deprecated since PHP 8.1 and so this PR updates our code to use FILTER_SANITIZE_SPECIAL_CHARS instead which is essentially the same thing.

Testing instructions

1. Switch your dev site over to PHP 8.2+
2. Enable Apple Pay/Google Pay
3. While on develop add a product to your cart and attempt to purchase it using Google Pay
4. When the Google Pay window appears, check you debug log and confirm the deprecated messages
5. Checkout this branch and confirm no deprecation messages
6. These changes impact fetching shipping options so confirm the shipping based on your address are correct

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

Fixes #3056

[a-danae]

Thanks for updating these, @mattallan! Things look good to me 👍

✅ No entries related to the deprecation of FILTER_SANITIZE_STRING are added to the debug log in this branch.
✅ The shipping options displayed in Google Pay match the ones defined in the WC settings for the selected shipping address.
✅ The customer details are correct in the Stripe payment details.
✅ The code changes make sense to me.
✅ I didn't find any other occurrences of FILTER_SANITIZE_STRING in the code.

Having the logs filled up by deprecated messages isn't nice. Glad to see this fixed!