Use `WC()->is_wc_admin_active()` to check for WC Admin note support

When trying to display the "DHL Express live rates are now available" note, [WCS&T currently checks](https://github.com/Automattic/woocommerce-services/pull/2402) if the following will throw an exception:

```php
try {
WC_Data_Store::load( 'admin-note' );
} catch ( Exception $e ) {
return false;
}
```

(For some context, the problem originally stems from the fact that WC Admin can be available as a plugin but can have its functionality disabled using a filter, in which case the `admin-note` data store does not exist).

While this currently does the job, @adrianduffell suggested [in this issue](https://github.com/woocommerce/woocommerce-admin/issues/6703) that `WC()->is_wc_admin_active()` can be used to check if we can add a note. The approach should be changed to be in line with this best practice.

**UPDATE:** `NoteTraits` now throws an exception if the store isn't available that can be caught instead of checking this but both solutions would work.

Start from the current WCS&T check that calls WC_Data_Store::load('admin-note') and review the linked pull request for its location. Compare that approach with WC()->is_wc_admin_active() and the NoteTraits exception behavior. Done means the note-support check follows the recommended WooCommerce Admin approach without breaking note display.

Written by the indexing model from the issue text.