[Backport to release/11.2] Revert apostrophe encoding in classic checkout requests - #69649

This PR is a cherry-pick of #69644 to release/11.2.

Original PR Description

Why

#68365 changed the classic checkout to send update_order_review and coupon requests as a pre-serialized string (encodeApostrophes( $.param( data ) )) instead of a data object. That has two side effects in 11.2.0.

1. $.param() turns fields removed via woocommerce_checkout_fields / woocommerce_billing_fields into empty values (country=&state=), where $.ajax() used to drop them, so the session customer's country, state and postcode get blanked. With billing_country removed and "Force shipping to the customer billing address" (or woocommerce_cart_needs_shipping_address false), place order fails with "Please enter an address to continue." for guests and logged-in customers. With postcode/state removed and "Hide shipping costs until an address is entered", shipping options disappear for logged-in customers. Reports: https://wordpress.org/support/topic/problems-with-checkout-after-last-update-to-11-2-0/ and https://wordpress.org/support/topic/shipping-method-disappears-in-classic-checkout-after-woocommerce-11-2-0/ (the first reporter's snippet follows our own checkout fields tutorial).
2. Third-party jQuery.ajaxPrefilter callbacks that read originalOptions.data as an object now get a string (e.g. HurryTimer's coupon expiry check, Global Seller Services).

Reverting restores the 11.1 behaviour. I reproduced it locally on 11.2.0: with only #68365's checkout.js hunks reversed, the blocked orders go through for guests and logged-in customers. The apostrophe/WAF fix from #68365 will be re-landed separately (passing the object to $.ajax and encoding in beforeSend), with tests for removed fields and prefilters.

What changed

• update_order_review, apply_coupon and remove_coupon pass the data object to $.ajax() again, so undefined (removed) fields are dropped and prefilters see an object in originalOptions.data.
• Place order sends $form.serialize() unchanged.
• The encodeApostrophes() helper is removed from checkout.js. The copy in cart.js from Fix apostrophe encoding in classic cart requests #68556 (11.3.0, not in 11.2) is untouched.
• The four Fix apostrophe encoding in classic checkout requests #68365 apostrophe tests are replaced with tests that assert the restored request shape, including country and state staying undefined when those fields are missing from the page. The mock setup that later tests (Fix checkout validation for non-error update notices #68360, Add the notices wrapper class to classic notices that rendered outside it #68716) rely on is kept.

Testing

1. On 11.2.0 add to a theme or mu-plugin: add_filter( 'woocommerce_billing_fields', function ( $f ) { unset( $f['billing_country'], $f['billing_state'] ); return $f; } ); and set WooCommerce > Settings > Shipping > Shipping destination to "Force shipping to the customer billing address".
2. Open the classic checkout ([woocommerce_checkout] page) with a physical product in the cart, fill the visible fields and place the order. It fails with "Please enter an address to continue."
3. With this branch, the order is placed.
4. In devtools, the update_order_review request no longer contains country= or state=.

Automated: pnpm test:js -- frontend/test/checkout in plugins/woocommerce/client/legacy passes (21 tests on trunk, 20 on a local cherry-pick to release/11.2, which applies without conflicts). eslint on checkout.js and the test file reports no errors.

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