[Feature] Have Installation Wizard default to server timezone

Currently the Installation Wizard defaults to `Africa/Abidjan` as it is the top-most entry in the list of timezones.

## Suggestion

Use PHP's [`date_default_timezone_get()`](https://www.php.net/manual/en/function.date-default-timezone-get.php) to check if the server has a timezone set and, if so, default to it in the timezone picker.