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