Fix default phone region validation

[Pytal]

Fix #35952

Summary

The default_phone_region config value

server/config/config.sample.php

Lines 203 to 211 in 74a0ac8

	/**
	* This sets the default region for phone numbers on your Nextcloud server,
	* using ISO 3166-1 country codes such as ``DE`` for Germany, ``FR`` for France, …
	* It is required to allow inserting phone numbers in the user profiles starting
	* without the country code (e.g. +49 for Germany).
	*
	* No default value!
	*/
	'default_phone_region' => 'GB',

was not being used when validating the phone number on the frontend

libphonenumber-js supports the ISO 3166-1 country codes as detailed in the docs https://github.com/catamphetamine/libphonenumber-js#country-code

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[Pytal]

/backport to stable25

[szaimen]

LGTM but didnt test