Migrate the remaining special_holidays to StaticHolidays class

[PPsyrius]

Proposed change

This PR migrates the remaining holidays that use special_holidays and its variants but are not yet migrated to the new StaticHolidays method, as done in #1509 and #1513.

List of affected countries:

• AO Angola
• BN Brunei
• KH Cambodia (already have this, but the method in question was mislabeled as CameroonStaticHolidays)
• CM Cameroon
• CN China
• SZ Eswatini
• ID Indonesia
• JP Japan
• KE Kenya
• LA Laos
• SK Slovakia
• KR South Korea
• UY Uruguay

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 added references to all holidays information sources used in this PR
• The code style looks good: make pre-commit command generates no changes
• All tests pass locally: make test, make tox (we strongly encourage adding tests to your code)

[coveralls]

Pull Request Test Coverage Report for Build 6590175977

• 79 of 79 (100.0%) changed or added relevant lines in 13 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.978%

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Totals
Change from base Build 6580894640:	0.0%
Covered Lines:	9987
Relevant Lines:	9987

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💛 - Coveralls

[arkid15r]

Thanks for refactoring this!

LGTM 👍