[sddm-theme/lock screen] Use QQC2 for the clock labels

Summary:
This patch replaces the PlasmaComponents labels in Clock.qml with QQC2 ones. This resolves the issue of the clock letters/numbers getting clipped.

BUG: 404651
FIXED-IN: 5.15.3

Test Plan:
Before and after
{F6680490}

Reviewers: #plasma, #vdg, filipf, ngraham, hein

Reviewed By: #plasma, #vdg, ngraham, hein

Subscribers: mart, davidedmundson, plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19647