RFC: Dropping Breeze + Oxygen Qt4 builds after

[David Edmundson]

We're starting to see a lot of preparation for Qt6 porting API which
we know will be deprecated.

One place where this is problematic is Breeze + Oxygen QStyles where
we currently provide Qt4 builds. I don't want this to hold us back and
be a problem later. It's also a problem for some shadow cleanup
happening now (D26475)

I propose that we completely drop the Qt4 builds from Breeze+Oxygen
after Plasma 5.18.

We're seeing distros phase Qt4 out, even Debian - and if a distro does
still want to package these, they will still have the option of
building the Plasma 5.18 tarball for Qt4 even after Plasma moves to a
new revision.

CC'ing packagers to give them a chance to shout.

David