Rewrite the painting of an month item

Summary:
+ Remove gradiant in background.
+ Simplify the drawing of the rounded rectangle.

Screenshot before: {F6309911}
Screenshot after:  {F6309814} {F6312595}

Reviewers: #kde_pim_junior_jobs, #kde_pim, mlaurent, dvratil

Reviewed By: #kde_pim_junior_jobs, #kde_pim, dvratil

Subscribers: dkurz, kde-pim

Tags: #kde_pim, #kde_pim_junior_jobs

Maniphest Tasks: T9420

Differential Revision: https://phabricator.kde.org/D16013