Update ISO3166 package to 4.1

[mikkamp]

Changes proposed in this Pull Request:

This PR updates the ISO3166 package to version 4.1, which removes any deprecation notices for PHP 8.1. The usage of the package remains the same so we don't need to adjust our code.

Note
Even though we are removing some deprecation notices, WP itself might still give other Deprecated notices for the Requests library, which are unrelated and need to be resolved in WP and/or WPCLI.
Unit test are also not ready to be run for PHP 8.1 (see further details here)

Closes #1745

Detailed test instructions:

1. Setup the plugin on a site with PHP 8.1
2. Send a request GET request to https://domain.test/wp-json/wc/gla/mc/target_audience (note onboarding must be completed)
3. Ensure we receive a response with a list of countries
4. If you are logging DEPRECATED notices to debug.log then check there before and after applying this PR, for the deprecated return type of ISO3166::count() or ISO3166::getIterator.

Changelog entry

• Update - ISO3166 package version 4.1

[jorgemd24]

Thanks, @mikkamp, the deprecated warning related to league/iso3166 are fixed 👍

As you mentioned in the issue: #1758 (comment) the warnings related to rmccue/requests are still present