Underneath the timezone selector on the General Settings screen, a small snippet of info about the selected time zone is displayed.
This information would be missing if the timezone is set to a deprecated timezone value, even though PHP is perfectly capable of generating that information, including for deprecated timezones.
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 condition from failing when the current timezone is a deprecated one.
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()].
Note: As this is an admin/output page, no pre-existing tests are available.
Follow-up to [54207], [54217], [54227], [54229], [54230], [54232], [54233].
Props jrf, costdev, marcyoast.
See #56468.
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