Eliminate use of date_default_timezone_set() for calculations in core

[Rarst]

Core sets PHP timezone to UTC on boot and assumes it for Date/Time operations. Which we are in process of improving to eliminate reliance on this assumption in #44491

However there are also couple places in core where it changes PHP time zone back and forth for calculations:

• in options to determine DST status
• in PHPMailer (which I assume is just inherited from upstream)

We should get rid of these, calculations should not need to rely on manipulating global PHP configuration state.

[SergeyBiryukov]

In 45556:

Date/Time: Eliminate use of date_default_timezone_set() for determining DST status on General Options screen.

Time calculations should not need to rely on manipulating global PHP configuration state.

Props Rarst.
Fixes #46483.