[Cuttlefish] Use only a single AppStream ID

Summary:
Right now Cuttlefish provides both org.kde.plasma.cuttlefish and
org.kde.cuttlefish.desktop AppStream IDs, causing it to appear twice in Discover and
other software center apps.

This patch standardizes on org.kde.plasma.cuttlefish, and provides a compatibility
shim so you can still access it from the old URL (appstream://org.kde.cuttlefish.desktop)

BUG: 411112
FIXED-IN: 5.16.5

Test Plan: Don't know how to test this

Reviewers: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23301