This fixes a bug where if the default `timezone_string` is set to a deprecated timezone name due to a localization providing an outdated timezone name string, this localized timezone string would be discarded and an empty string would be set as the timezone value instead.
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 ticket #56468.
Also see: [https://www.php.net/manual/en/datetimezone.listidentifiers.php PHP Manual: timezone_identifiers_list()].
Includes:
* Expanding the translators comment to encourage translators to use “old” names over “new” names.
* Adding a dedicated test to the `Tests_Admin_IncludesSchema` test class.
Follow-up to [54207], [54217], [54227], [54229], [54230].
Props jrf, costdev.
See #56468.
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