Update Japanese holidays

Summary:
This patch is purely cosmetic since the predicted Vernal and Autumnal Equinox Days of 2021 are the same as the official announcement.
The source URLs in the comment section are now in HTTPS. (When using the HTTP version, a 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