au: Add National Day of Mourning for Her Majesty the Queen

[danielwhite]

This is a one-off public holiday for 2022 that will be observed by all of the states and territories.

The Prime Minister has declared that Thursday 22 September 2022 will be a public holiday for a National Day of Mourning for Her Majesty the Queen.

Source: https://www.fairwork.gov.au/newsroom/news/national-day-mourning-and-other-upcoming-public-holidays