fix(material/datepicker): avoid showing month label twice in calendar

[PreethiPantangi]

fix(material/datepicker): avoid showing month label twice in calendar

MatCalendar always renders a header that displays the current month/year. MatMonthView can also render the month label inside the calendar grid when there are enough leading empty cells, causing the month to be displayed and announced twice. The in-grid label can't be made aria-hidden, as this affects NVDA's weekday column count.

Fix
Add a showLabel input to MatMonthView, defaulting to true to preserve the behavior of standalone @docs-private usages. MatCalendar passes false since its header already displays the month/year. I also ran pnpm approve-api datepicker, updating the golden API files for the datepicker package.

Fixes #33763

Testing:
pnpm test datepicker

Before

Date.Picker.Before.mp4

After

Date.Picker.After.mp4

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