[applets/digital-clock] Fix date sizing in vertical panel

Summary:
This patch fixes several errors in the code:
- The property is named `font.minimumPixelSize`, not `minimumPixelSize`
- The calculation in `minimumPixelSize` did not make sense
- Setting the height to `dateLabel.paintedHeight` is nonsensical

BUG: 417852
FIXED-IN: 5.18.4

Test Plan: {F8186873}

Reviewers: #vdg, #plasma, cblack, davidedmundson

Reviewed By: #vdg, #plasma, cblack, davidedmundson

Subscribers: davidedmundson, cblack, muesli, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28172