Deprecate system_time_zones() and move to TimeZoneFormHelper

Problem/Motivation

Much of the logic in system_time_zones() is outdated, as PHP now has the native ability to only return current timezones. While we're modernizing this logic, it makes sense to deprecate and remove (in Drupal 9) the old function.

Proposed resolution

Switch from using a regex to filter out old formats, and simply use \DateTimeZone::listIdentifiers(). At the same time, mark the old function deprecated and move this logic either to DateHelper. Note, since this relies on Drupal\Core\StringTranslation\TranslatableMarkup, it can't go into a component such as DateTimePlus.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet