Fix selectors for quantity element in variable products

[clustercis]

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Closes #36855 to fix bug introduced by PR #36087.
Also addresses original issue #36086.

Edit 2023-02-20 (added details from commit message):
WooCommerce 7.4.0 changed the jQuery selectors for the quantity input in variable products.

This PR reinstates the previous logic so that $qty points to the div wrapper, and $qty_input points to the input element inside.

Otherwise, as is in the affected version, $qty_input is undefined and the min/max attributes are not updated.

It also addresses the original issue #36086 by restricting the selector of the div wrapper to only be the one containing input[name=quantity] (the .qty class filter is also kept to avoid any potential malfunctions of existing code).

How to test the changes in this Pull Request:

• Have you followed the Writing high-quality testing instructions guide?

Quantity selector works properly (adapted from steps to reproduce in #36855):

1. Create a variable product with some variations.
2. Set a variation to "manage stock".
3. Set the variation's "Stock quantity".
4. Set the variation to "Do not allow" backorders.
5. View the product from the front end.
6. Pick the variation.
7. Try increasing the quantity higher than what you set the stock quantity to.
8. The input should now enforce the max limit so the value should not change above the set quantity.

Quantity selector does not affect other inputs (adapted from testing instructions in #36087):

Add the following snippet, which adds a new quantity input to the single variation:

function kia_test_single_variation_qty() {
	echo '<div class="quantity"><input class="qty" type="number" name="tacos" /></div>';
}
add_action( 'woocommerce_before_single_variation', 'kia_test_single_variation_qty' );

1. Go to the variable product above.
2. Select a variation by selecting the attributes necessary.
3. Increase the rogue quantity input's value.
4. Change to another variation.
5. The main quantity input's value should stay the same.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you created a changelog file for each project being changed, ie pnpm --filter=<project> changelog add?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

cc @barryhughes

[codecov]

Codecov Report

Merging #36871 (db2986d) into trunk (2e146d5) will increase coverage by 0.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##             trunk   #36871   +/-   ##
========================================
  Coverage     46.7%    46.7%           
  Complexity   17171    17171           
========================================
  Files          429      429           
  Lines        64762    64762           
========================================
+ Hits         30220    30222    +2     
+ Misses       34542    34540    -2     

Impacted Files	Coverage Δ
plugins/woocommerce/includes/class-wc-tax.php	79.0% <0.0%> (+0.5%)	⬆️