Stop using QVariant < operator

It will be removed in Qt 6 without porting strategy:
f43cb31ba0

We know that that variants will be either ints (for `count`) or longs (for `size`),
so just convert them to longs (to avoid overflows) and compare those.

src/kitemviews/kfileitemmodel.cpp

@@ -1771,7 +1771,7 @@ int KFileItemModel::sortRoleCompare(const ItemData* a, const ItemData* b, const
             } else if (valueB.isNull()) {
                 result = +1;
             } else {
-                if (valueA < valueB) {
+                if (valueA.toLongLong() < valueB.toLongLong()) {
                     return -1;
                 } else {
                     return +1;