[sddm-theme/lock screen] Adjust login screen and lock screen font sizes

Summary: This patch increases the font size of the username and action button labels for better readability.

Test Plan:
Before:
{F6645900}
After:
{F6646411}

For reference, level 4:
{F6645901}

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: filipf, ngraham, plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19433