Update Thailand holidays: refactor Royal Ploughing Ceremony code

[PPsyrius]

Proposed change

As แรกนาขวัญ (Royal Ploughing Ceremony) entries are all manually assigned, MAY, 13 fallback date is no longer needed.

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 (12f2602) to head (ef22e4d).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #2247   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          196       196           
  Lines        11891     11891           
  Branches      1717      1717           
=========================================
  Hits         11891     11891           

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[KJhellico]

LGTM.

[arkid15r]

LGTM 👍