Add Easter Sunday to AU-SA public holiday list

[bbayu123]

According to the South Australian Public Holidays Act 2023 - which has gone into effect since 1st January 2024, replacing the Holidays Act 1910 - Easter Sunday is observed as a public holiday.

Easter Sunday needs to be added to the AU-SA holiday list starting from 2024.

References:

1. Public holidays (SafeWork SA), https://www.safework.sa.gov.au/resources/public-holidays
2. Public Holidays Act 2023, https://www.legislation.sa.gov.au/lz?path=%2FC%2FA%2FPublic%20Holidays%20Act%202023

[KJhellico]

I'm working on a large update for Australia holidays (inspired by #1674). @bbayu123 , thank you for your suggestion, it will be useful.