In Manjaro (and other Arch-based distributions) you cannot update only one component of the system, because this will lead to an unsupported partial update state.
Update your whole system, like so…
sudo pacman -Syu
The conflicts you are seeing right now are probably the result of an earlier partial update, and as I said, this is not supported. What you could try — I don’t know whether it will work — is reinstall plasma-workspace and possibly kate.