Use the new MONTH_IN_SECONDS constant in human_time_diff()

[tyxla]

#33397 introduces a new constant MONTH_IN_SECONDS, which is defined as 30 * DAY_IN_SECONDS.

The human_time_diff() function uses the month time in its logic as well, and it is also currently using a hardcoded 30 * DAY_IN_SECONDS value for months.

So for consistency we should replace all occurrences of 30 * DAY_IN_SECONDS to MONTH_IN_SECONDS in human_time_diff().

[tyxla]

Use the new MONTH_IN_SECONDS constant for the month representation in the human_time_diff() logic.

[ramiy]

+1

[SergeyBiryukov]

In 35555:

Use the MONTH_IN_SECONDS constant added in [33698] for the month representation in human_time_diff() logic.

Props tyxla.
Fixes #34602.