Shift for parts of the CI system to Qt 5.15

[Ben Cooksley]

Hi all,

This weekend parts of our CI system shifted to using Qt 5.15, with all
FreeBSD builds now being based on Qt 5.15. We also shifted all Linux
builds of Plasma, and the latest Qt version build of Frameworks to Qt
5.15 as well (apologies for the massive amount of email this kicked
up)

It has however exposed a series of SIC changes in Qt which will need
to be adapted to. The list of affected projects appears to be as
follows:
- kalarm
- kdesdk-kioslaves
- kompare
- kaffeine
- subtitlecomposer

In addition, the following projects appear to have long standing build
issues. It would be appreciated if they could please investigate and
correct these:
- kbibtex
- atcore
- kmymoney
- ring-kde

Further, the following project appears to have a broken build due to
SIC changes within another KDE project:
- zanshin

Cheers,
Ben