Set the range of possible pruning size in the widget before loading its value.
Without this thee value would get clipped to the default spinbox range 0--99.
Remove the upper-bound limit from pruning size setting.
Hypothetically, someone may wish to begin pruning at a future blockchain size, and there's no reason to limit it lower.
Backport of Core PR15801
This includes some changes from PR15163 (use of header guiconstants.h)