Use new folder-stash icon

Summary:
Resolves T7185

Use the new folder-stash icon introduced in https://cgit.kde.org/breeze-icons.git/commit/?id=b909a8008a7e3ffabb090bc82127ebb561f58597

Test Plan:
Tested in KDE Neon. Dolphin now uses the new icon is kio-stash is installed:

Reviewers: elvisangelaccio, Dolphin

Reviewed By: elvisangelaccio, Dolphin

Subscribers: cfeck

Maniphest Tasks: T7185

Differential Revision: https://phabricator.kde.org/D8722