Check whether South Korea's Constitution Day got a substitute holidays from 2027 onwards

From #3284 

> The substitute holiday (대체 휴일) for Constitution Day is still being pursued by the Ministry of Personnel Management (인사혁신처) and hasn't been officially enacted yet, so I'll hold off on adding `append_observed` and updating the docstring for now. Will add it in a follow-up PR once it's confirmed.