Proposed change

Update Australia holidays: fix Sovereign's Birthday dates in WA.

According to WA Public and Bank Holidays Act 1972, celebration day for the Anniversary of the Birthday of the Reigning Sovereign to be appointed for each year by proclamation published in the Government Gazette (this is usually the last Monday in September, but not always).

Closes #1756.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new python-holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've run make pre-commit, it didn't generate any changes
• I've run make test, all tests passed locally