fix: setting defaultMonth the next year when using the dropdown mode does not render the calendar

[rodgobbi]

Fix #2776

Description

The issue happens because the valid months of the calendar are calculated internally in the useCalendar hook when the layout of the day picker is a dropdown type, but other inner logic parts of the day picker use the startMonth and endMonth props as the calendar limits, which in this use case is undefined.

Changes

This PR unifies the source of truth for the start and end months of the day picker, using the navStart and navEnd returned by the getNavMonths in the useCalendar hook as the source of truth.
This also means that from now on, we should NOT use the startMonth and endMonth props directly, and instead use the navStart and navEnd values.

[gpbl]

Well done @rodgobbi, thanks!