Convert krunner plugin to Qt6

I've noticed that in e93f1a31 krunner plugin has been restricted to KF5. Not sure if that was temporary or an intentional decision to stop plugin's development. In any case I've tried converting it to KF6 and it builds in https://invent.kde.org/mischurow/skrooge/-/jobs/2241548 (the job as a whole fails but for unrelated reasons). This MR contains a single commit from that branch with only changes under plugins/.

Unfortunately, it doesn't directly build under KF5 due to change in namespace, but it doesn't seem like something that couldn't be fixed with enough ifdefs. Alternatively, this could wait until master is swithed to KF6 (d2017fd5 will have to be dropped from MR though).