Set correct start date for 2x Historical South Africa holidays

[Firefishy]

Proposed change

South African historical holiday's improvements:

• Union Day started in 1910, the year of the South African Union.
• King's Birthday from 1910.

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/pin/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've successfully run make check, all checks and tests are green

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (47010d3) to head (5c5ecb2).

Additional details and impacted files

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##               dev     #2276   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          200       200           
  Lines        12181     12181           
  Branches      1731      1731           
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  Hits         12181     12181           

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[PPsyrius]

South Africa's holidays cut-off start date is already set as 1910 though so this might be redundant:

    # Observed since 1910, with a few name changes
    start_year = 1910

[Firefishy]

As pointed out by @PPsyrius this PR is redundant :-)