Sunsetting Qt5 support

[David Edmundson]

> We need to decide what to do with these right now. I see:
> - Keep them around but without CI coverage? (my least favourite option for obvious reasons)
> - drop them for Plasma 6.7 and tell distros if they need them build them from the 6.6 tar ball
> - keep them and have some CI setup where we build them against a KF5 stack from distro packages (if possible)
>
> Did I overlook some other option?

We should do option 2.

David