refactor: replace time() with Time into date_helper and Email class

[ping-yee]

Description
See #6815

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[ping-yee]

This is off topic, I have a question that what function in time class replace the microtime() in system/Debug/Toolbar/Collectors/Database.php is more appropriate?

[kenjis]

Since PHP 7.1, DateTimeImmutable has microseconds when constructed from the current time, either explicitly or with a relative string (e.g. "first day of next month").
See https://www.php.net/manual/en/migration71.incompatible.php#migration71.incompatible.datetime-microseconds

We can use u in format:
See https://www.php.net/manual/en/datetime.format.php

[MGatner]

Looks good!