sanitize_option(): fix bug in sanitization of timezone_string

This fixes a bug where if the `timezone_string` would be set to timezone name which has since been deprecated, the option value would be "lost" when saving the value again, as the comparison being done to verify whether it is a valid timezone name would only take "current" timezone names into account and would invalidate deprecated timezone names.

By passing the `DateTimeZone::ALL_WITH_BC` constant as the `$timezoneGroup` parameter to the PHP native `timezone_identifiers_list()` function, a timezone name list is retrieved containing both current and deprecated timezone names, preventing the invalidation of the option value.

See the extensive write-up about this in Trac#56468.

Also see: https://www.php.net/manual/en/datetimezone.listidentifiers.php

Includes adding a dedicated test to the data provider used in the `Tests_Option_SanitizeOption` test class.

Note: I have made this a _named data set_, even though the other data sets are unnamed, to make sure it is clear what this data set is testing.

Adding test names for the original data sets in this data provider would be a great future improvement, but is outside of the scope of this commit/ticket.

Author: jrfnl

src/wp-includes/formatting.php

@@ -4923,7 +4923,7 @@ function sanitize_option( $option, $value ) {
 			break;
 
 		case 'timezone_string':
-			$allowed_zones = timezone_identifiers_list();
+			$allowed_zones = timezone_identifiers_list( DateTimeZone::ALL_WITH_BC );
 			if ( ! in_array( $value, $allowed_zones, true ) && ! empty( $value ) ) {
 				$error = __( 'The timezone you have entered is not valid. Please select a valid timezone.' );
 			}

tests/phpunit/tests/option/sanitizeOption.php

@@ -69,6 +69,7 @@ public function sanitize_option_provider() {
 			array( 'timezone_string', 0, 0 ),
 			array( 'timezone_string', 'Europe/London', 'Europe/London' ),
 			array( 'timezone_string', get_option( 'timezone_string' ), 'invalid' ),
+			'Deprecated timezone string is accepted as valid (Trac#56468)' => array( 'timezone_string', 'America/Buenos_Aires', 'America/Buenos_Aires' ),
 			array( 'permalink_structure', '', '' ),
 			array( 'permalink_structure', '/%year%/%20%postname%', '/%year%/ %postname%' ),
 			array( 'default_role', 'subscriber', 'subscriber' ),