Update Japanese holidays

Summary:
This patch is purely cosmetic since the predicted Vernal and Autumnal Equinox Days of 2021 are the same as the [[ https://eco.mtk.nao.ac.jp/koyomi/yoko/2021/rekiyou211.html | official announcement ]].
The source URLs in the comment section are now in HTTPS. (When using the [[ http://www8.cao.go.jp/chosei/shukujitsu/gaiyou.html | HTTP version ]], a [[ http://wwwa.cao.go.jp/notice/20191101notice.html | notice on "always-on" TLS of Cabinet Office Website ]] will be displayed.)

Reviewers: dvratil, winterz, cgiboudeaux

Reviewed By: winterz, cgiboudeaux

Subscribers: ltoscano, aacid, #kde_pim, kde-frameworks-devel

Tags: #kde_pim, #frameworks

Differential Revision: https://phabricator.kde.org/D27419