[plasma-workspace] Use new activities icon

Summary:
Use the new icon added in D22155.
{F6948276}
{F6948277}

Test Plan: Open the desktop context menu and also search for "activity" in KRunner.

Reviewers: #plasma, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22167