fix: Time::getAge() calculation

[kenjis]

Description

• fix Time::getAge() calculation
• fix test case

Returns the age, in years, of between the Time’s instance and the current time. Perfect for checking the age of someone based on their birthday:
https://codeigniter4.github.io/CodeIgniter4/libraries/time.html#getage

There are a few ways to calculate human age.
However, the current implementation does not seem to be common.

E.g.:

• birthdate: December 31, 2020
• now: January 1, 2022
• age: 2

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

[MGatner]

Why not use Time::difference()? Or it's underlying method:

   return $time->fieldDifference($this->testTime, IntlCalendar::FIELD_YEAR);

[kenjis]

Refactored.

[MGatner]

Test failures seemed to be due to CloudFlare's outage affecting Coveralls, not a problem.

[samsonasik]

Thank you @kenjis