System: refactor birthday calculation logics and functions

[yookoala]

Description

• Mark daysUntilNextBirthday and is_leap_year deprecated.
• A new Format::daysUntilNextBirthday protected method to replace daysUntilNextBirthday() in functions.php. The new method is re-implemented with PHP core's \DateTime.

Motivation and Context

• The function daysUntilNextBirthday() is only used by the Format class.
• The function is_leap_year() is only used by daysUntilNextBirthday().
• The logics are based on explicit leap year calculation, which is also done internally with PHP's own DateTime implementation.
• Functions that do not needed to be public should be kept as implementation details within a class.

How Has This Been Tested?

• Locally.