Change "different user" icon on login screen

Summary:
Replace "different user" icon on SDDM login screen to match icon used on
unlock screen. Original icon was `system-search`, now changed to `system-switch-user`.

BUG: 392830

Test Plan:
- Recompile `plasma-workspace`
- Log out of current session
- Check that `Different User` icon is now a figure with an arrow

Reviewers: ngraham, #vdg, #plasma, davidedmundson

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

Subscribers: abetts, sebas, davidedmundson, #vdg, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12036