[FIX] website_sale: State select readonly in shipping info

If we take the below facts:
 - The country select is set as disabled
   when Shipping is set to "Ship to the same address"
 - The disable property of select inputs is
   removed when shipping is set to
   `create a new address`:
   In `website_sale.js`:
   ```
   $selects.attr("disabled", value <= 0 ? null : "disabled" ).prop("disabled", value <= 0 ? null : "disabled" );
   ```

We can safely assume that the select input "State / Province" was supposed
to be set as `disabled` in the first place, not as `readonly`

Before this revision, State / Provice was greyed when choosing
"Create a new address" for shipping, and selecting the United States
(but the select input was still usable, though, it was just greyed)

opw-675739

addons/website_sale/views/templates.xml

@@ -960,7 +960,7 @@
                   </div>
                   <div t-attf-class="form-group #{error.get('shipping_state_id') and 'has-error' or ''} col-lg-6">
                       <label class="control-label" for="shipping_state_id" style="font-weight: normal">State / Province</label>
-                      <select name="shipping_state_id" class="form-control" t-att-readonly="  'readonly' if shipping_id &gt;= 0 else ''">
+                      <select name="shipping_state_id" class="form-control" t-att-disabled="  'disabled' if shipping_id &gt;= 0 else ''">
                           <option value="">State / Province...</option>
                           <t t-foreach="states or []" t-as="state">
                               <option t-att-value="state.id" style="display:none;" t-att-data-country_id="state.country_id.id" t-att-selected="state.id == checkout.get('shipping_state_id')"><t t-esc="state.name"/></option>