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• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The proposed changes allow the taxes endpoint to return an array of postcodes/cities for tax rates with multiple (semi-colon separated) postcodes and/or cities in two new keys, cities and postcodes; the old existing city and postcode keys are kept for compatibility and will return only one of the available values.

Closes #27750 .

How to test the changes in this Pull Request:

1. In WooCommerce admin go to Settings - Tax - Standard Rates and create 3 rates: one with no cities or postcodes specified, another one with one city and one postcode, and one with two cities and two postcodes (separate them with a semicolon, ;).

2. Temporarily add this at the ned of your woocommerce.php file, this way you won't have to bother about REST API authentication: add_filter('woocommerce_rest_check_permissions', function() {return true;}, 10, 0 ); (note that this will cause a bunch of unit tests to fail)

3. Retrieve the taxes via the /wp-json/wc/v3/taxes endpoint and verity that the results are consistent:

For no city and no postcode:

 {
    "postcode": "",
    "city": "",
    "postcodes": [],
    "cities": []
}

For one city and postcode:

 {
    "postcode": "1111",
    "city": "OSAKA",
    "postcodes": [
      "1111"
    ],
    "cities": [
      "OSAKA"
    ],
}

For two cities and postcodes:

{
    "postcode": "2222",
    "city": "KOBE",
    "postcodes": [
      "1111",
      "2222"
    ],
    "cities": [
      "OSAKA",
      "KOBE"
    ],

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 successfully run tests with your changes locally?

Changelog entry

Fix - Add array-typed "cities" and "postcodes" to the response of the "tax" endpoint in the REST API to overcome the limitation of "city" and "postcode" returning always one single value.