Update UK Early May bank holiday for 2020

Summary:
Reference: UK bank holidays - GOV.UK
Special thanks: Ian Neal, who reported the bug and suggested the change

BUG: 409189

Reviewers: KDE PIM, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: iann, dvratil, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D22099