user-manager repo is unmaintained and obsolete for Plasma 5.20 and later

[Nate Graham]

Hello distro packagers!

This is a heads-up that the user-manager repo 
(https://invent.kde.org/plasma/user-manager) repo is unmaintained and 
obsolete as of Plasma 5.20 and later. Starting with Plasma 5.20, the 
users KCM has been moved into the plasma-desktop repo, so the 
user-manager repo can safely be removed from users' systems.

Nate